Nanny extra hours - not paid yet.?

I dont know if anyone out there can help me: I work for almost 2 years for a family as a Live In Nanny.

I have a question that I would like to know if you can answer for me. PLEASE!

I went on holiday with the family I work for and I was doing only nights 5pm to 8am because they have a baby under 6 months. The parents went away and I stayed with the children in the grandparent's house.

Many times during the day I helped the grandparents because they were visibly struggling with the kids around, I mentioned this in an email to my boss and she replied "Why don't you help them out as needed this week. Track your hours and we'll give you extra holiday time when we get back to London."

The total amount of extra hours I worked was 2 days and 2 hours. My boss wants to pay back the extra time owed by taking it off the babysitting I do twice a week which is included in my salary.

I think is unfair. :-(

So my question is: Do I have any obligation to accept or should I ask for the extra holiday she mentioned in the email?

Thanks for you help

Hebe