@LavenderBush
It makes a difference how long you've been staying there. Cleaning it on the first or second day might be a bit iffy, but cleaning it after you've been there a week has more of a "pitching in and helping clear up the mess I've helped to create" vibe.
The fact that the OP said this:
We are currently staying at mil house while we have some building work done. I had a shower this morning and noticed that the shower head was grim with limescale.
suggests that it was nothing to do with them staying, and pitching in to help. She clearly thought the bathroom was not clean enough for her, as soon as she got there.
I think it is very rude. There are very few things more rude, than someone thinking they know what's best, and insisting on doing things for you, that you never asked them to do. 
In real life, the posters on here who claim they wouldn't mind, and are saying 'can you come do MY bathroom,' would be pissed off. I think anyone would if someone came to their house and started cleaning it. How rude. 