Many thanks for that information. I will check those electrical connections for corrosion and use a multimeter to see if there is any significant resistance. One battery reads 11.6V but the other reads 12.6V and I normally run with the switch at the '1+2' position, so I am not sure how much voltage the pump would actually get in normal running - maybe it would only be the lower of the two, i.e. 11.6V?
I suspect that the fuel filter would only show clean fuel since I have twice changed it since this problem arose. At first the engine would not rev over about 2000 rpm, but after I changed the filter and pumped the tank to filter out the rubbish it would then rev to about 3000 rpm. So I changed the filter again hoping that it would rev fully, but there seems to have been no further improvement. Still, maybe I could check the contents of the filter again.
I suspect that the fuel filter would only show clean fuel since I have twice changed it since this problem arose. At first the engine would not rev over about 2000 rpm, but after I changed the filter and pumped the tank to filter out the rubbish it would then rev to about 3000 rpm. So I changed the filter again hoping that it would rev fully, but there seems to have been no further improvement. Still, maybe I could check the contents of the filter again.