sorry for taking ages to reply
the solution in the end was simple!
i took the car to a local garage and told him what national tyres had said
he said he doubted it was the lambda sensor
he started the car...said "can you hear that"?
i couldnt hear anything!
he showed me that the exhaust was blowing a little
he did the emissions test...it failed (no surprise)
patched up a hole in the back box...it passed!
so got a new one fitted £53 inc fitting
took it back to national tyres for re-test...passed
i want to thank you all here for all your help....
you all been very very helpful