I dont know anything about the laws regarding bait digging on SSSI sites or any beach but i do recall there being big issues up in the North East regarding bait digging a few years back. I had a quick google and found this, hope it helps.
http://www.independent.co.uk/news/uk/court-sets-legal-right-to-dig-for-worms-on-shore-1564955.html

 

I just found this on the net.

THE BAITING GAME: Digging bait for personal use - like the holidaymakers pictured here - is fine but large-scale ventures could be far more damaging to the environment around Poole harbour

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ILLEGAL bait-digging teams are targeting the mudflats of Poole harbour, causing "environmental vandalism" to the area's eco-system.

And as in the cocklepickers tragedy in Morecombe Bay earlier this year, the commercial diggers could be putting themselves and others at risk, it is claimed.

It is thought some are travelling from more than 50 miles away to seek lugworms in Poole.

There is nothing to stop fishermen digging for bait for personal use - but the harbour appears to have attracted a larger scale illegal commercial venture.

Cars and vans have been spotted parked, with bait boxes being filled by groups of diggers.

Investigations are under way and Dorset Police's wildlife officer PC John Snellin said: "It does seem as though this is not just bait digging for private use. There have been reports of cars and vans with bait boxes."

He is liaising with other harbour agencies to address the problem and believes there are safety issues, such as potential heavy metal contamination in the water.

Hamworthy councillors Roger Gregory and Mike White are so worried they have raised the alarm at what they claim is a problem of "epidemic proportions" and have brought the matter to the attention of Poole Harbour Commissioners, English Nature and the RSPB in a bid to safeguard parts of the harbour designated sites of special scientific interest (SSSI).

They claim diggers can now be seen from the bird-watching hides in Upton Park and are in danger of putting herons, egrets and other wading birds off nesting next year.

Cllr White said: "This is environmental vandalism from profiteering bait diggers."

Cllr Gregory added: "We have sent out an SOS for our SSSI. It is essential that this illegal digging is stopped, and stopped quickly."

And Joan Wright, chair of the Harbourside Residents' Association, said: "Up to nine or 10 (bait diggers) each day are parking on the estate, using scales to grade the worms and leaving sand and worm bits in the road.

"The area used to be visited by flocks of godwits, redshanks, and other wading birds but numbers are now sadly depleted."

Helen Powell, English Nature conservation officer, said: "We are concerned about the reports we have received regarding the level of bait digging in Holes Bay. The public right to dig for bait is limited to personal use only - it does not extend to digging on a commercial scale.

"Poole Harbour is a Site of Special Scientific Interest and it is also internationally important for its inter-tidal habitats and the large numbers of wintering birds that it supports.

"We are working with the police because we have a number of concerns about the damage and disturbance being caused at the moment. Last year a code of conduct for bait diggers was produced which also set out the legal position on the collection of bait."

As far as I am aware Bait digging in an SSSI is dependent on local policies and you would need to check for any restrictions in your area.

 

Found this in a document about a North Linconshire SSSI:-


Starting just south of Grimsby Dock Gates, the coast changes to an area of mud and sand. This opens the area up to other activities, such as dog walking, bait digging and sea fishing. The Beach Safety Team polices bait digging. There are areas where bait digging is permitted: north of Wonderland and south of Cleethorpes Leisure Centre. Digging on the saltmarsh is not permitted.
A licence is required to bait dig in the designated areas; this is available from the Tourist Information Office. With the licence is a code of conduct that is issued to all bait diggers. Appendix 6 presents a copy of this leaflet.

 

The only thing that is clear here, is that designation of an area as a Site of Special Scientific Interest, does not in itself mean there is a ban on bait collection.

Prevention of any activity would need to be covered by some other legislation or local bye-law.

I regularly dig in an SSSI, and there is nothing to prevent me doing so.

It may be that digging worms has absolutely no effect on the original reason for creating the SSSI in the first place.

It is a common and recurring problem in this country, that once somewhere is given any form of environmental protection status, then some people think it means the area should be entirely sterile of any activity.

Pennog.

 

I also dig in SSSI's. Unless there is a specific local byelaw that prevent's you from digging, which I doubt there will be, (but make sure!), you should be alright to carry on digging.

 

Admittedly a good few years ago now but at one time I handled the liaison between local council, english nature and our local angling club when our local estuary (the River Colne) was declared a SSSI ... I was a professional digger at the time.

Things may have changed a little since but as the law stood at the time you could not neccesarily be denied a legal right to dig bait on an SSSI unless it damaged the species or geographical features specifically protected under the SSSI or some other form of legal restriction was put in place. I also learnt (from the then English Nature surprisingly) that when an SSSI is declared the relevant owner of the land (in our case the local council) has to agree a management plan including all the activities that will take place on the land they own.

The interesting point at the time was that if the council forgot to include say, angling, then you weren't breaking the law so long as you didn't damage anything listed as protected under the SSSI, in fact it would be the landowner that got their wrists slapped.

On the Colne angling and bait digging were not restricted in any way with the new SSSI but I would suggest checking out what the terms of the SSSI there were and what (if any) restrictions were put in place as many councils are actually managing them illegally in my opinion (and it is only my opinion), believing that not including angling and bait digging in their management plan immediately brings a ban into effect - I'm pretty sure that isn't the case.

Also worth remembering though is that even if the bait species itself (lug rag whatever) are not notified (ie protected specifically) and digging bait is not specifically listed as prohibited you could still be breaching the law gathering them by either damaging a "geographical feature" or by accidentially disturbing other species in the process which are notified.

The easy option to finding out if digging and angling is allowed is an email to Natural England, they should be able to tell you exactly what is protected and prohibited on any SSSI. If you have a look at the Natural England website there's a weeks worth of reading on the subject and you should even be able to find the name of the NE officer responsible for your site.

http://www.naturalengland.org.uk/ourwork/conservation/designatedareas/sssi/default.aspx