Thank you for the messages.
Removed carbs today, really clean no sediment etc. Blown though everything anyway.
I then checked the pilot screws and No 4 o'ring was snapped. So that looks like the problem.
I think No 3 was just not getting as hot as 1 & 2 because of No 4 problem.
Fingers crossed a new o'ring will sort it out.
Put it all back together running on four now and then fuel leaking out of both overflows!
Out they came again and checked float heights and needles all good.
Back together and fuel still coming out of both overflow pipes?
Got set of needle seats with new o rings - this has fixed it.
It turns out that one pilot jet o ring and two needle seat o rings (one one each bank!) all failed at the same time.
Moral of the story - if you remove the carbs its worth getting a set of o rings etc and changing the whole lot.
Good news is, it is running better than ever now with much smoother pull from lower down the rev range