Here is the happy ending!
I had to resort to brutal methods to remove the pilot screw. I sawed off the 'tube' around the pilot screw to get better access. I tried tapping it with a hammer and punch to encourage it to unscrew, I tried allsorts but to no avail. I ended up drilling through the screw with a cordless drill, starting off with a very small dia drill and gradually increasing the drill diameter until i was as close to the ali thread as possible. Using a sharp "V" needle file inserted in the drill hole I cut through the wall of the brass to weaken the remains of the screw. The brass is quite crumbly. I tapped the top of the thread with a scriber and pin hammer and winkled the remains of the screw out. There was some damage to the thread in the carb body, but I was able to clean this up with an M6 x 0.5 mm second tap (thanks for the tip Sirc).
The guys at NRP
www.nrp-carbs.co.uk were unbelievable helpful. They had the spares I needed, AND suggested that I had further probs with the choke plungers (I could not use the choke when starting as the engine would completely flood). I took their advice, fitted the new choke plungers, new pilot screw, new float valves and gaskets. I reset the float height, set the pilot screws at 1.75 turns and put it all together
The bike is now excellent. Starts immediately, ticks over well, and runs very well. No misfire, no hesitation, smooth and lively and is now a joy to ride.
Thanks guys for all your help