Nope. The little bugger won't come out. I tried a pointy thing, a fine but strong slot-head screwdriver, I tried knocking a slot into the top of the remaining stud, all to no avail.
However, I've just taken the bike out for a spin and it works perfectly ok with 4 springs, although the clutch is very light, which, in London is not necessarily a bad thing
Bike will be put into the workshop soon to have this sorted and the seat-unit can be painted at the same time.
Thanks for all the advice everyone