Trust me to have an oddball reel. Unlike the Shimano's the kit is made for: mine has the body and endplate machined from a solid billet of alloy.
Which meant the endplate was 3/8" thick, same as the length of the threaded bush.
Russ, at blakdog, helped enormously, answering each query straight away.
I ended up counterboring the end plate to accommodate the bush and spacer and fitting the spring to the outside, there not being enough room for it between the endplate and spool.
Tried it this morning and it is magic: I can cast for the first time with a multi and remain birdsnest free.
My life is transformed.... Many thanks to Blakdog for all the help.
Don.