Thought I'd pop on an answer (albeit a bit late) as I'm a pet sitter myself : )
I'd say that £8 a visit is absolutely fine - especially since she has no petrol costs - but this is a fair price in that you know she'll do the job responsibly for a decent pay. As she's coming twice a day, it's easy for her to keep on top of the hutch and the cat's litter tray etc.
Why not leave out a present or send a thank you card afterwards - or have a chat with her along the lines of 'have you thought about increasing your prices as we think your service is really good value for money' then gauge her reaction. If she's happy with the rates it should be obvious
We have a cat and 2 guinea pigs and are going away for approx. 16 days soon. We usually pay our pet sitter 16.00 a day for 2 visits. She feeds our cat (dry food) and spends a bit of time giving her a brush and cuddle. She also feeds the guinea pigs and cleans out their hutch regularly. Before we had the GPs we paid 6.00 for each visit to our cat so we've added 4.00 for the extra work. Do others think this is a fair rate - or should it be increased? She usually cycles to our house - she's about 15 minutes away. I don't know anyone else with pets that pay for a sitter.