/I had the same thought regarding backnuts and washers. Depending on the
quality of the surfaces, could you not use a fibre washer to seal?

Otherwise araldite? I'm assuming you'll never want to get them apart
again! You could file the flats off afterwards to reduce required hole
size in worktop?

As Brian suggested, could you not exchange the taps for ones of the
right length of shank?/q

The radiator extenders don't even have flats... but I think a different tap is the best avenue, shame it took long enough to find this one! The only way I can see extenders working will leave me either trying to bodge the securing of the taps to the worktop, or bodging the seal of the extenders onto the taps' tails, both will most likely entail frigging abt whittling wood off under worktops or trying to redrill/resize the existing holes..... how would I even do that?

**** it new tap!

Cheers

Jim K