I've got an idea..........

Ok I'm going to balance the carbs again and then take it out...

Oh I completely missed in your first post that you said carbs were balanced
But how long ago was that? As I had the same problem on my bike where one of the downpipes didn't heat up properly on tickover and balancing cured it, even though they'd been balanced about a year before (not by me though so possibly not done well enough).

I balanced them about 200 miles ago and they certainly needed it, before that it would hardly run at all, but no harm in just eliminating this from the equation so will check it again.

Still cannot understand why leaning it in (turning the mixture screw clockwise) is causing that cyclinder to fire when screwing it out (rich) is stopping it from firing on idle. could it be providing so much fuel that it's flooding the spark?

Could it be...the idle mix screw has the tip of it broken off? The little brass screw has an almost needle like tip on it, if that has broken off then it may be running crazy rich on 2.5 turns out, then once you screw it in, it leans it back to something approaching normal?

Failing that have you still got the little rubber o-rings on the idle mix screws?
Ive had trouble before when a couple of them were missing, caused some really odd running.

As jac says, also you could replace the pilot screw o-rings as they're old, that's something I did recently actually. My local DIY shop had a box of assorted o-rings, they need to be 3mm OD 1mm ID 1mm thick iirc.

Checked the needles and they're fine. Where do the o-rings sit?

On each pilot screw should be a spring, a washer and then the o-ring - often the o-ring doesn't come out with the screw but stays inside the carb so you'll need to poke it out or tap the carbs.

Problem solved - transpired it had no o-ring at all!, now running fine at 2.5 turns - thanks guys