In message , writes
I have a 'washbasin wrench' which can be persuaded to get hold of the
union nuts but there's no way I can get enough purchase in the
wrench's handle to actually undo the nuts.

If the wash basin wrench is gripping the nut OK try locking a pair of
mole grips onto the shaft/tommy bar to give a bit more levereage. If
that's doesn't shift it, use a stilson wrench to turn the tap itself[1]
using the basin spanner and mole wrench to stop the coupling nut
turning, you can often get the mole wrench to press up against the wall
to stop it turning.

[1] assuming you don't want to reuse it

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bof at bof dot me dot uk