I've acctually got my problems solved now!!!
The stupid petrol cap turned out to be only a small part of the problem (I got a proper Alfa one for £9 anyway!) it was the Lambda Probe in the Exhaust that was shot, also the guy in the garage noticed that the connection to one of the plugs was a bit iffy...
I got a new probe fitted, and the guy replaced the cable for free because he had one lying about. She's now driving beautifully, a truly wonderful car and amazing for what it is (1.8)