We have heated water buckets over here - are they available in the UK yet? If not then I can post a link but you would need to use these buckets with an adapter.
put them in the back corner of the stable in the horses bed (we leave some deep litter around the bucket). Keeps the temperate around the water a bit warmer.
sorry forgot to add, I use one of those big soft rubber trugs buckets and it has been ice free so far. We are in an open barn stable though. I think its warmer than being in an outside stable.