I finally managed to get the fuel pump check valve to undo from the banjo fitting on the fuel line by manipulating a 12" shifting spanner over the body of the check valve to hold it steady while undoing the banjo fitting. This enabled me to get the pump assembly out of the tank. The rubber part of the pickup under the pump itself has totally decayed and broken into two pieces as you can see from the pics I've uploaded to here:
I suspect but still don't know for certain if that decayed rubber pickup is the whole cause of the car's engine running and starting inconsistencies but having proven the electrical side it 100 percent ok, it seems 99.9 percent likely to be the sole cause!
Will post more when I know for sure.
Craig.