Hello Bonsaichris,
Your Gardner sounds like the one thing better than a Lister in a narrowboat! Ref the HA2, not personally familiar with the model, but if it is similar to other aircooled Listers, there are a few reasons why fuel ends up in the sump, as I have discovered. Assume the injection pumps are behind a sideplate in a compartment connected to the crankcase. You may be able to remove rocker covers and visually check while running that you don't have obvious leakage from injector lines or leak off pipes. More difficult to run the motor with the sidecover off to check pump connections, but worth a try while cold, and at low revs. this may show a leak. The fuel supply line has banjo connections to the pumps, with bonded or "Dowty" washers either side of them. Check or replace these first, and if that is not the cure, wipe the top of the pump body dry and look for leakage where the delivery valve holder screws into it. Using 2 OE spanners, loosen the pipe nut, carefully try tightening the valve body in the pump, (not too tight) then re tighten the pipe nut. Also helps to slightly loosen the pipe nut on the injector end, then tighten each a little at a time to ensure both ends align evenly. If there is still a leak under the valve holder, there is a fibre seal ring that succumbs to old age and over-tightening. The pumps need removal to replace this, best done by a fuel shop, as the seal is not sold as a separate spare. Be sure to retain the correct timimg shims under each pump. Hope some of this helps, and applies to your HA2, and is not too confusing. (afterthought,) amateur wrench jockeys can ruin the cone ends of pipes by misaligning and overtightening, so look at them too. Regards, Combustor.