could be the ECT (engine coolant temp) sensor tellin gthe ecu that its warm when its not , about 15-20 quid from woodford garage and the likes.

a quick check... its resistance when cold (20 dgc) should be 3.75Kohm, droppin gto 0.38Kohm when at 80deg(normal runing temp)

the ect is the small blue sensor underneath the inlet manifold at the back of the engine - bit of a tw@t to get to and it scres into the head with a copper washer sealing it to the head .