I dropped the tank on Saturday and found that the carbon filter on the smallest vent pipe was blocked (couldn't blow through it either way). I am under the impression that this is the vent pipe for allowing air into the tank when the car is running (with the small pipe on the filler neck being for venting during filling only). I took this filter off so that the tank now vents straight to atmosphere (i'm assuming this filter is only there to protect the environment and serves no mechanical purpose).

I have also replaced the fuel pump and wiring and the car is now running sweet as a nut so I'm hoping this is problem solved.

I haven't filled the car up at a petrol station yet so I'm not sure if my filling problems have been fixed by doing this as well. I have read on the internet that some cars have problems with venting during filling (MGTF's and Porsches) so I'm not sure whether this could just be down to poor tank design (thanks to my cheap aftermarket tank) or whether I still have more work ahead of me.