Many thanks to James and Paul for the excellent advice. Sorry to post this reply so so late, but for the record, here is what seems to have worked so far. I did not do the work myself, instead using the excellent mechanic who does our Audi work as well as other side work on English classics. Having also read the forum here, and cleaned out years of gunk, he couldn't see why there were two breather caps closely adjacent on the injector pump. When the smaller of the two caps was unavailable for replacement, he suggested and I agreed that we just cap it off with a bolt. See photos of the two different sized breather caps and the bolt now taking the place of one of them. The car is a 1964 RHD but the engine is 1968, and the pump is PES6K1 70B120 R20Y, it also has the numbers 0408 026 017 on the bottom as well as 80420924 on the top of the label. In any case, the car has been faultless all summer, though has only about 50 miles on it since this repair. So far, so good.