Personally less than two thousand £ would be my guess.
Do you have a V5? does it run? does it even turn over?
If you have none of these that devalues it a lot.
Its a nice bike but not that special a bike, not rare at all and for that price you can get a very well set up ready to ride bike.
My advice to you is to either bung it on ebay and let it find its own value, or sell it via Kettering Classics who handle a lot of bikes for the owners club. They wills sell on commisssion for you.
My own personal pessimistic valuation, as a long term Matchless/AJS owner,on a bike in this state with no current V5 and a non runner is £1000-1200.
Why? because to put it back on the road overhauled but unrestored will cost that much again.
Mediocre old bikes are not the gold mine many people think they are. Except for the spares suppliers...It'll need new rims and ryres for a start, 5-600£, if the engine is stuffed, and it'll certainly be in need of a major overhaul, big end £200, then you need to get it fitted, piston £50, a set of cables 50quid if you can't make your own, it mounts up alarmingly.
not so many years ago a bike like this would have been broken for parts.
good luck with it.