Rob wrote
This is perhaps not quite a simple as it seems !

Some idiot got into the garden last night and emptied something into
the pond which obviously has a high detergent content as we returned
from a night away to a pond covered in deep foam stirred up by the
pump. I haven't discovered yet what damage has occurred to the plants
and fauna.

I've effectively pumped out the pond now and done one refill aiming to
pump it out again tomorrow with the hope that two flushes plus a bit of
sunshine will clear the problem, but realised that I now have a good
opportunity to address the over zealous lilies in the pond - actually
that's probably unfair to the lilies as I gather that they do expand
fairly vigorously and 'pruning' is necessary every so often, though
possibly not in mid summer !

But with a nearly empty pond and huge amounts of lily growth I think I
would be daft not to take advantage and do some tidying up - but the
question is how ? The pond is nominally 10 ft diameter and 3 ft deep
and the root mass of the two lilies is pretty large - do I just go in
with a spade and reduce the plant mass ? ... or what ?

Anybody in the Ediburgh area want some water lilies ?

It's actually best done when they are in growth anyway unlike most land
plants so it should be Ok.
Yes, go in with a spade and chop out what you don't want, mind the liner if
it's lined, you may find the best newest most vigorous growths are outside
any pot so you will probably have to repot those and throw away the old
stuff. Filthy job! :-)
What variety/colour are they?

--
Regards
Bob Hobden
17mls W. of London.UK