Got round to looking at the bike again today.
New switch is ok so disconnected the banjo feed to one of the rockers and started engine - no oil getting to rocker. Hmm!
Drain oil, sump off, clean out all oil ways as best as I could dismantled and cleaned pressure relief valve, fit new filter, clean gauze filter, refit sump with new gasket (dry - no gasket goo), refilled with 3 litres of Mobils finest and guess what - oil pressure light now goes out almost immediately engine starts - bingo!
Didn’t have time to take it for a run but tried stopping / starting engine several times and it looks like it’s fixed.
Thanks for all your help guys and I was hoping to be able to tell you what it was but it’s a complete mystery to me. Maybe you’ve got some ideas?
Rich