I was going to suggest that too!
Switch works at very low psi so should be quite feasible to make it go out on the starter in normal circumstances.
Memory is sketchy but sure I’ve seen it happen, oil light goes out but charge light stays on.
???
Reading the thread again I see I already suggested that earlier. 
Waiting reply from OP…
You did.
Curious one this, I can’t think what might have gone wrong.
Just a thought, what oil did you put in?
Have you rechecked the level?
It is odd.
Almost like he forgot to refill it!
I’ve heard of people who forget to put the sump plug back in and it ends up all over the garage floor!
I was thinking a lightweight oil may flow too easily and not allow the pressure to build up.
Try putting your multimeter across the switch to see if it changes state when engine starts.
Hi Don
Yes I’ll try that when I get a minute. Probably won’t be till next weekend now.
Thanks
Hi Don
Mobil 15/40 which I’ve used before, First thing I did was check the oil level but plenty in there. Had a thought about a blockage somewhere + the pressure relief valve in the sump not working.
Just seems really weird it’s only since changing the oil. Will try taking off the rocker feed next weekend to see if any things getting through.
Could there be an air lock causing the pump not to prime properly?
Hi
Haven’t tried this but get the same result with a brand new switch I just fitted.
Sounds to me as if the pressure relief valve is stuck open.Collapsed spring??a piece of debris??Or that the oil pump drive isn’t driving.Loose sprocket nut, sheared woodruff key?? I’d take out the switch and quickly turn the motor over to see if oil squirts out of the hole.
Doh that’s the other thing you can do of course.
Don’t get air locks just by draining and refilling.
Even when sump off, all washed out and new filter, so all galleries and filter completely empty, still doesn’t stop it working, so no.
It could be a duff new switch.
as has been posted remove it and turn the motor over.
If oil squirts out borrow someone elses to confirm it it not the switch then check your relief valve.
They can stick open after a while.
Ok thanks. I was just throwing ideas out there
Excuse my stupid question- but the pump is at the top of the engine so must work on vacuum/suction from the sump to get oil up there before it can be pumped around at pressure?
Thanks Ian will try what you suggest at the weekend.
Sorry ol’ chap but if your oil pump is at the top of the engine then that’s your problem.Everybody else has theirs at the bottom. 
Oops! Must be reading the Haynes manual upside down Just assumed pressure switch was in the pump housing. Doh!
It’s below the crankshaft sprocket, at the front, and driven by same via the cam drive chain.
EDIT: so not too far from the sump to suck up the oil.
Any more developments ???
OK check switch with multimeter. If OK remove sump. Take old gasket off and sling it. Clean joint face spotless. Fit new gasket no goo. Check filter seats correctly try new filter. As you say the bike was fine before you did ‘x’ job therefore it is more likely to be x. Cranking engine over on starter with oil switch removed will show oil pump is fine. Don’t need to turn ignition on.
Only other thing I can think of, gauze pick-up strainer is blocked - but that’s a helluva blockage… (big mesh area)