Can I do this? got a couple of spare 100Watt heaters
that I used in my indoor aquariums.
Its winter here in the UK and I'm a bit worried about my
pond fish.got 5 golden orfe's and 3 regular goldfish in it.
The pond is approx 40 gallons with a pump/filter setup,
and is above ground level.

It would be cheaper I think to just buy a stock tank heater and use
that, instead of the heaters you are speaking of.

A floating stock tank heater can be bought at any farm store, and it
is thermostatically controlled.