Saracen Fuel Pump! yep they ARE a bit of a sod to get at, - offside air filter has to come off, ideally, as this will give you more light on the subject.
As for removing it, you'll need a shorter spanner than normal, 1/2 " AF, if I recall, which after loosening, you'll get about a flat at a time turning, both nuts all the way off the studs before pump will come away, WORD OF WARNING!!!!!- the pump push rod may fall out into the depths of the engine compartment!, so be ready with your third hand to catch it!.
Marcus Glenn, and Champ Spares sell new old stock pump reair kits at about £45, well worth the money, but a fairly head scratching-ly fiddly job!
I had the same symptoms as you described, did my pump, and it improved things no end, my old beast has since been clocked at 55mph!
If you have any problems, feel free to get in touch!