KS, you must be a cheap part of Surrey! I'm paying £8 per hour for evening babysitting... not that I'm complaining mind, as the children adore our babysitter.
Loulie, finding a reliable babysitter is tricky. Found out latest via the web, not something everyone would feel happy about I expect, but it's worked well for us, as we have a qualified and experienced nursery nurse caring for our children, and the children don't mind at all about being left. Also it's handy having a babysitter who will just take over the bedtime routine at whatever point it's at - sometimes not even started!
Back in the 80's when I did babysitting as a teenager, like you I put ads in newagents windows. These days I don't see many of those sort of ads appearing. Now we all have the net... I tend to use that for most things, and it's not only the big agencies that use the web, we found an independent babysitter via searching on google.
Ok so our babysitter isn't a teenager... not sure how to find teenage babysitters. Anyone got any ideas? Word of mouth is probably best I guess.
To answer the origional question... daytime care isn't really babysitting, it's nannying. In my area, £10 per hour would be typical for ad-hoc nanny work like you describe. So 2.5 hours, I'd expect would cost £25 to £30, depending on minimum charges... after all if someone is coming some distance, then 2.5 hours isn't a very long time for daycare.