OK - time to report back.
It's a single outlet tap; I'm not sure I know where the second outlet would link to?
The NRP kit arrived well; you don't get a lot for your £25 (cone, two screws and a couple of rubber seals. The spring fits between the lever and the cone, which pushes the cone into it housing which is logical even if it leaves a seemingly pointless void at its other end.
It was a tight fit and when assembled, bound the lever. The cone is, I would judge, turned from stock and had a small burr on the larger end, which I then removed hoping it would press home. But the tap still passed fuel when in the off position! Neither could I find space to fit the large washer so it got replaced by an O ring. This large washer prevent a petrol leaking out of the tap - it doesn't help seal the on/off/reserve function., so that's not the problem.
Petrol must be passing between the cone and its housing. The two seem to fit reasonably well - but maybe not as snug as they should be. Whether that is because the new cone has an incorrect taper or the tap is worn, I'm not sure.
Anyway, somewhat frustrated, I had another go at moulding a new cone. It's not perfectly petrol tight, but it's the best yet. I think the problem is with the two ports that lead to the tank. When moulded two limbs are formed which need to be cut off perfectly flush.
