Cheers Paul! I'll be in touch about the ECU.
When I first checked the loom, it was still in the car and hence a bit awkward - I removed it at the weekend, and it tested up fine. Was a bit disappointed that I'd not found the problem. Obviously, when I put it back in, the symptoms persisted.
I need to get a timing light on it really - when the rpms fluctuate, I suspect that the timing is retarding and advancing.