PLEASE HELP. Sorry about the caps but this is urgent.

Please help.
We have a pond, about 3 x 4 x 4 (feet). We've had it for about 5 years
and everything was great. It contains about 40 goldfish but recently
they have begun dying. The dead ones are small and appear to be a
silvery \gold type colour.
We don't do anything to the pond other than feed the fish and clear
off the leaves and stuff off the surface and until now all has been
fine. We haven't changed the food nor put anything in the water. The
bigger fish seem to be OK but 5 have died in the last week.
There is no residue or anything usual about the pond. The fish are a
bit slow but that is just the cold (It's winter here England,
Brighton)
Could it be something to do with the fluctuating temperature we're
experiencing this winter?
Please, if you can help or recommend anything we'd be most grateful.
Thanks.