Put another 5 litres in, if that doesn't do it, you've got a poorly pump or a restriction somewhere. I'd be trying to blow through the pipe at various points.
Pleased to say it was just low on fuel (gauge doesn't work so it's difficult to tell, and given the car hasn't moved in a long while I wouldn't be expecting it to be dry). Thank you all for your help/ suggestions.
I've topped it up and checked there's fuel coming through the return.
Tried to start her again and it's hesitating, feeling very much timing related.
Also managed to cook my negative to earth wire in the process, suggesting a loose connection somewhere. Progress? I think so!