wrote in message .co.uk...
Management is concerned that the goldfish in the pond are, as last year,
going to die if the pond is frozen over for anything more than a few
hours,

Well,,,, maybe "management" knows more about it than
I do (certainly wouldn't be hard !).
But in my experience, in a climate generally similar
to the UK (USA PacificNW), that in winter my
ponds would sometimes freeze over for 7-10-14 days,
and when naturally thawed, the goldfish were still
fine.