Hah. Sorted. Found my relays, swapped a brand new one in, then headed off to the nearest petrol station, 14 miles away. A couple of hiccups but massively better. Got 18.2 litres into the 18l tank, so figure I was pretty much replacing all fuel. A few coughs as I headed back but, by the time I got home, it was running just fine - I even got to find out where the shift light is. So I'm provisionally saying, 80% relay, 20% fuel. The other possibility is corrosion on the TPS pot that's just worn off with use.
