Hi John, thanks for your advice, the bike was last ridden 2 weeks ago, and certainly no evidence of wet sumping, never found more than a smallcup ful of oil in the sump.Just had the sump plate off complete, really unsure if the ball was stuck, cleaned out throughly with carb cleaner, then noticed when inserting the mesh filter plate the gasket was on the wrong face and realised I must have assembled it and covered the ball tube, and that's been like it since recommissioning in April, so hopefully issue solved, bike started easily abd ran first for 5 mins, oil return fine, removed sump plug and maybe egg cup worth of oil came out.
Need to get some fresh fuel and take her on a run.
It's the 2nd school boy error of late, I was struggling with the Tickle, took ages, then finally discovered the float was back to front, ie float is marked clearly front, never noticed that before on my Amal carb