First thing to do is replace any old rubber fuel lines in the system, if problem persists, pressure check the steel feed line for pinholes, if all OK there then your problem is in the fuel pump.
First thing is to install an in-line tap in the fuel feed line on the end of the steel line in the engine bay, or hidden away somewhere protected but still accessable to you.
Problem will go away as fuel can't drain back to the tank anymore, just don't forget to open the tap if you want to get more than 100 yards!
This will solve the problem in the short term, but remember your fuel pump is on borrowed time. The simplest answer is then to replace the mechanical pump you've got with a new one. Only if you have fuel pressure issues would I change for an electric job with an adjustable regulator (something cheap for an injected 3 litre would easily do). Just remember to ditch the mechanical one at the same time as all its doing is breaking up and blocking your filter.
The problem I believe you have is that the one way valves on either side of the diaphragm in the fuel pump are worn and are allowing fuel to drain back into the tank when the engine is not running.
Those sealed unit mech pumps are OK but the big aluminium jobs with the removable cover and filter are great, (well I think thery are, 'cos you can strip 'em in the field next time this happens).
Good Luck.
Now, gimmie another Guru point!
