thanks for your responses. Limp Mode is restricting the revs to around 2500-3000. All sensors and all things mechanical have been checked and replaced and are all working fine. The ECU was replaced with one from a scrapyard around a year ago, and part of the connection is bound by black - after much checking and replacing of parts etc. by an alfa expert, it looks like the ecu wiring is dodgy causing this side-effect. I didn't even know the ecu had been replaced until some investigations by the alfa expert shone light on this fact - turns out it is a small world after all!
Is the 2500 potential min revs likely to impact the MOT testing Iain mentioned?
Thanks again - helps me to know what I'm talking about when I choose and speak with MOT tester.