How often should someone I’m renting a holiday house from come to check the pool?

It is the law - they are required to test every day.

Testing the pH to see if you'd peed in it maybe, lol

Certainly it invades your privacy but it's just what these owners want. They must be leery of renting out their place and looking for an excuse to come by frequently and make sure their house is being kept carefully by you. There's probably some local rule requiring someone to check the pH frequently, but maybe it doesn't have to be them. If you're enjoying your holiday, there's no sense getting into an argument, however.

If your kids peed in the pool or the inflitration became faulty, would you want bacteria with a risk of sepsis or any other disease.

they are covering their ****, as people love to sue.

UK

rnaybe you can offer to check that for thern

Unless one has been doing it a while, pools need daily checks/possible adjustments. After one is used to how their particular pool runs, they can skip a day or two here and there.. If there is rain, unusually hot weather, more usage then normal, then one needs to go back to testing and adjusting daily. For problem pools, sometimes multiple times a day.

I'd expect to have the house to myself too. That seems weird, and definitely more like them keeping an eye on the premises and who you have with you. I've never heard of such a "legal requirement," but I'm not familiar with Devon, perhaps it's different there? (Which I doubt, but I don't know).

I'd certainly include that in my review of the accommodations.

Any pool that is used daily will need it's water tested and chemicals adjusted otherwise it could become unsanitary. That is the "by the book" safe way to deal with the pool so a law wouldn't surprise me. You should be grateful that they are taking care of this for you. I've had rentals where they just leave a note with a closet of pool chemicals for you to figure out. Otherwise I'm sure they would rather not have to come every day either, but it's the right way to deal with a pool. I realize many people probably wouldn't be so vigilant, but if you really knew what was lurking in those pools you wouldn't question it so much.

Once or twice a week is plenty. It does feel invasive and unacceptable.

You do have the house to yourself. The pool is outside, right?

It takes five minutes. And you're free to request a specific time of day if there is one that is better for you.

It is recommended to check pool pH every other day, but more often if recent adjustments have been necessary.