So, thinking about it more, (and sitting here with a glass of Shepherd Neames’ finest) I might have done something wrong!
I just replaced the tank and pipes to it like for like. As I said I had to cut a couple of connectors, but I’ve replaced the pipework.
The tank that came off had the (correct?) fuel return, the one on top of the tank, blanked off with a piece of hose and a bolt. The fuel returned to the pipe that comes out of the top, front, right hand corner of the tank, so, I did the same on the new tank.
I wonder if, in the old tank, that pipe extended down to the bottom of the tank, as the centre one does? Can’t check now as it’s away to the scrappies.
If that’s the issue, what’s this pipe at the front for ?