Water Temperature & Heating

How dificult is it to sustain a water temperature of atleast above 10C but average to 15C?

Is it hugely expensive? Is it dificult to sustain a steady temp?

can be difficult depending on your heater and pond positioning. If the pond is exposed to the elements then you will be fighting against windchill - during late autumn to spring your pond will need to be covered and pipework and filters insulated.
Gas is by far the cheapest way to maintain heat - however, if like me you are limited to electricity then yes it is expensive - a word of warning don't opt for a profi heater I did and regretted it - nearly boiled my fish in the garage because the thermostat decided it wasn't going to work anymore (apparently a common fault with profis)

Like Richard has said with good insulation you should be able to maintain a reasonable temperature. The best object of heating is to eliminate the big fluctuations between day and night temps.

I was thinking of getting a profi simply for the cost savings - not necessarily a long term solution. Is it worth paying more than twice the price for an elecro?

I am currently exploring connecting up to my central heating system - since the pond will only be 6ft from the house. But no idea how to get this to work... any experience? (wiring, zone valves and plumbing all giving me a headache!)

My main aim is to stabilise temps, and keep them over 11/12 through winter.

I know it can be done - I will try and find the details for you.
The profi is known to be faulty so it would be false economy - you can pick up cheap elecros on ebay