Sounds like a partially blocked tank breather to me, or a collapsing feed pipe. On tickover there isn't sufficient suction to cause a problem. As the engine revs increase the suction on the feed pipe can be so great it causes the pipe to collapse in on itself. A similar effect would be caused if the tank breather is blocked then the tank won't allow oil down the feed pipe. Try taking the oil tank cap off and/or temporarily replace the feed pipe with an original old one.
Thanks for that. I'm still on it's case but have been diverted recently with electrical work on my Honda XR250 enduro. I couldn't understand why the new LED indicators didn't work until I realised it's system didn't convert the alternator's output from Vac to Vdc and the lights ran on Vac. The LED flasher unit only 'likes' Vdc. Now sorted but the garage is too cold to approach the BSA!!!
Alan
