[phil_h]

I think you have to change one thing at a time now, starting with swapping the plugs from front to rear, to prove that each pair of items will work on the rear cylinder - even a new plug can be duff.
You havnt mentioned the carbs - have they been apart ?
If so, are both the diaphragms working properly (you can test by hand by carefully pushing them against the springs) cos that may well stop one starting (!)
Have you flushed the float bowls to make sure its not water making the plug wet ?
If you flushed the float bowls was there any muck - cos that might have blocked the pilot jets.
Have you tried a squirt of special starting-juice (cant remember what its called) into the front when the back is turning the engine over ?

time for bed now
good luck
phil
btw - where are you in case someone wants to come round ?