How much to tip cleaner?

I have recently started having a cleaner for 2 hours a week. I work full time and realised something had to give as I have health issues and was beginning to struggle. She is through an agency and the cost is £25 for two hours. I am guessing she is on minimum wage or thereabouts. She is divorced, lives alone and works very many hours in what is a physical job, so I think she probably doesn’t have much spare cash. I was planning on giving a card, box of chocolates or similar and a £25 tip as a thank you and Merry Christmas. But never having a cleaner before I have no idea what the going rate is. Does this seem reasonable? TIA