My bike starts but is not running properly on cylinders 2 & 3 - have sprayed WD40 on the down pipes for these two cylinders and they're definitely not at the same temperature (they're colder) than cylinders 1 & 4. Cleaned out the carbs but the problem persists, I've also tried fuel down the carbs of cylinders 2 & 3 but it made no difference. Is it likely to be the plugs?

Any advice gratefully received.

plugs would be a good start

You can swap the working plugs to the dead cylinders and see if the problem is reversed

Welcome to the site..

Chucking fuel down the cylinders can quickly kill the plugs if they weren't dead already!

2 & 3 (Front Left / Right Rear?)

There fed by different coils right? In that case its not likely to be coil / plug lead issue.

As already been said though, plugs are fragile, so a new set first.