Had new bathroom fitted about a month ago, all was working well, now the hot water both upstairs and downstairs is only luke warm.
Ant suggestions

You didn't say but I assume this is a electric water heater. Several things could cause this problem.
1. A faulty element **OR**
2, A broken dip tube see image)
If you need more help click on back. Good luck, Tom

Nope sorry, it's a gas boiler heatng system, all the radiators are hot, but the water isn't?

So, because you failed to give me all the information to answer your question you gave me a "Not helpful"? Thanks a lot. It's people like you that makes me wonder sometimes if it's all worth it. Perhaps one of the other experts can better help you.