Hi BN. To be perfectly honest, I don't think your wage is that low, considering you have your own flat. It's not high, but it isn't THAT low. However, I think the issue here is more how they treat you. From previous posts I don't think they are really very nice to you. Things like, don't touch the barbecue when we are not here speak volumes about how much they value and trust you.
I think it sounds like they are struggling to pay you, and trying to squueze the most work out of the least pay. Of course, this isn't really your problem -- especially if they are going to treat you the way they do. I mean why should you care if they don't?
I think if you found another live in job for the same money, you would probably be happier. Perhaps people who have had a live in nanny before and know how to treat one.
Where are you? (I'm sure you've said before but can't remember -- sorry)
What is the notice period in the contract? I would think this notice applies to a change in the contract as well as terminating it. So, if you have a month month notice period,m then they have to wait at least one month to implement the change (IF you choose to agree to it).
If I were you, I would put my CV together, and apply for some jobs. See what comes up. If you get a better offer, take it.