Since the engine has been stood for a few years with old fuel in & I'm now having issues with cyl's 2 & 3 not firing (I've tried swapping the coils over & cleaning the plugs) so Im thinking about stripping down the carbs. Can I get away without using the rebuild kits if I just want to clean out the jets, etc or do I need to fork out the £60 or so for 4 kits?

I wouldnt spend any money, full strip, clean with petrol and compressed air.
If diaghrams are split - replace.
float the floats in water to make sure theyre not leaking.
rebuild and set jets/floats to factory tolerences.
re fit