Thanks Judderbar. I wouldn't say that I know what I'm doing but I am very keen to learn! Are there any resources you could point me to that would help me to identify where the problem is?
The local garage had another look at it yesterday and managed to get a better diagnosis; the error message I've been told was 'random multiple misfire on coil 1'. They've said that they'd recommend taking it to a Fiat garage as they'll have better diagnostics kit. I'm keen to try to sort it myself if possible though as the nearest Fiat garage is quite a drive away and the 2-cylinder misfire is no longer intermittent. Don't want to risk causing further damage by driving it a distance...
I do not know of any online resources you could try.
Do you have a voltmeter?
Are you able to connect two wires to a 75W light so you can use it as a test light and connect the voltmeter across the light? Connect one end of the light to the battery negative terminal - the ground
Check the brightness of the bulb and the volts measured on the coils wires and compare the result you get with the good coil.
Do not turn over the engine.
If you do this properly you will be able to get brightness or volts for the fusebox side of the coil and the same measure for the ECU side of the coil and be doing that when the wiring can pass a 6amp current thru the bulb.
Report back.