OK, so just now I did the last bit of work for the tail lights - at least until I can confidently identify bright enough blade type reversing lamps! - by swapping out the amber indicators.
What had stopped me doing this sooner was that the batch of ten lamps were the correct single pole 2-lug base but the lugs on the original incandescent lamps are offset!
So I put them away muttering about reordering the correct type. Well, just now I decided that reordering is unnecessary when you have a Dremel.
I converted two of the new LED lamps into single lug bases and they're in and flashing away happily.
No need for resistors,
@CraigB , just like the rest of the lamps thanks to the eBay-sourced adjustable timing blinker relay designed not to care.
So next time you have lugs that don't match, grind them off!