I'm not sure where you're going to get one for £3.6ish, every where i looked they were 5 grand, except Bill Higham, who according to reports will not honour the web prices.
It was at Bill Higham - can't find the ad again now - when I got Tohatsu, I took the best 'boat show' price (NEC not London) to my local dealer - he first said he couldn't do it, then phoned the next day to say he could.
So don't know if you could do this will Bill Higham's prices as armour - perhaps not if it's known they don't honour advertised prices - but if this is the way they operate, why doesn't someone report them to the ASA or Trading Standards?
£5k does sound a lot for a 50hp though - Extreme Marine are advertising them at just under £4k - but that's starting to get a long way from the £2.5k of the OP even if it does rise to £3k if the grommet has to be done.
I appreciate the £2.5k is for a new engine and an old leg, but its used value will based on it being a 2005 engine and my real point was that you're talking a one year warranty verses a five year warranty.
This bit also has me worried "This motor was sold to a sailing club and had very little use" why would a sailing club buy a 40hp engine, only reason I can think of is for a safety boat and that would get plenty of use.
Our local club has 5 safety boats - three of which are in continuous use the other two of which are used occasionally for a larger event - having said that, the 'reserves' are the older, smaller boats - perhaps if a wealthy club has a similar situation, their reserve could be a new engine? - easiest way to verify the story is of course to ask who the club was and to contact them - if the dealer won't say, then perhaps there is a dodgy aspect to the story.