I did it with the mixer tap and it's working fine now I'm really happy thank you! You could hear it all gurgling back round the pipes. I will try not to let my dad turn valves off without turning them back on in future!
I hesitated posting earlier because I thought I had fixed the problem - turns out I haven't.
I ran the bath this morning and it filled up as normal but towards the end the hot water pressure became very low until water stopped coming out completely.
I turned the hot water heating off (gas...