Thanks again for your help, after a struggle I managed to release the fitting after dissolving the limescale. It did involve a lot of twisting and turning, the collar kept coming up with the pipe. The good news is that on inspection it appears that the ice pushed out the hoses a little way, past the pin that holds them. Having pushed the pin out, they went fully in with the help of a smear of silicone grease. I haven't refitted them back to the basin yet, but they're air tight so I'm confident they're water tight.
Once I've refitted that one, the kitchen tap is similarly leaking. That'll be a bigger job as it involves removing the fridge - not looking forward to that.
I knew about the need to leave the taps open, but I was too late - the water tank and boiler tank drained ok, but the taps were already frozen and wouldn't move. Lesson learnt.
I'm very grateful for the help on here, I didn't want to force anything without knowing what type of fitting I was looking at.