Final fettling requires running the engine, obviously. But I kept running out of fuel, and a lot faster than I should. The first 5l lasted a while, the second 5l not really long at all. It didn't seem to be leaking out anywhere so I wondered whether the problem was with the fuel pump. I had already bought a replacement, so I put that in. While doing this I noticed that someone had put two in line filters in at some stage, one between the tank and the pump, and the other past the pump. Looking at them, the one from the tank looked to be full of...well...mud. I pulled it out and replaced it and mud came out of the fuel hose. Taking off the fuel tank top and looking in revealed the source of the mud; lots of rust in the fuel tank. Despite advice to the contrary I'd avoided taking the tank out as it is a bit of a pest and I figured that if the rest of the car was rust free well then so was the fuel tank. Wrong.
Taking out the tank actually isn't that hard, or it wouldn't have been if the captive nut on the most inaccessable mounting bolt hadn't been broken off in the past. In the end I had to drill the top off it which took about an hour. Tank removal otherwise took around 15 mins.
I have fashioned a plate with the captive nut bolted into it, and riveted it into the bracket. The inside of the tank is a bit rusty but not terrible. I'll get it cleaned out and have it back in soon.

You can see from the image below how superbly inaccessible the bolt is. I think the captive nut was sheared off by accident damage; there is a fair bit of bog in the panels at the rear of the car.