The bike fires up straight away, even pushing the kick starter by hand, it does seem to fire on the top cylinder, but very intermittantly until warm.

To be honest I cant see how a crank bearing is going to stop it firing on one cylinder, then again, I'm more at home (dare I say it) with a 4-stroke engine!

Thanks guys for all the pointers, I'll give them all a try. Any more idea's though, keep em comming!