Hey all, Sorted the problem. Two of the three wires going to the throttle position sensor were connected the wrong way round and the sensor was bad so the mechanic said.
He swapped the wires round and added a new sensor, now I got proper 250 power!!! Only now a friend wanted a 2005 Suzuki RM250, and as mines as new condition he took it for a spin and bought it off me.
So this afternoon I went to my local (local as in 50 miles away!) Yamaha dealer and bought the 2006 YZ250. I put an hour of steady riding on the engine earlier and can't wait to let it have it tomorrow.
Thanks for everyones support in sorting this problem, You all made it a whole lot easier and I've learn't a lot from this experience.
Many Thanks,
Ashley,
Any feedback on the 2006 Yamaha YZ250 would be most appreciated.