nvj, I got my first aupair (French) off greataupair. One week before she was due to arrive, she told me she had unexpected change of plans! But I think I was unlucky. If you are thinking of great aupair, lots of mnetters use www.aupairworld.net - they are pretty much the same in how they operate.
Can't comment on gumtree as I have never used it but also mentioned here a lot.
Kitty is right about the aupairs on those sites are largely not yet in UK. But because I get 100s of responses (not so much my profile, which is wonderful of course, but because I happen to live in London where they all want to be), I can narrow down to the few who have previous aupair experience in UK (important for checkable references) and/or who are already in UK.
My present aupair (German) is lovely, so 2nd time lucky, and she was already in the UK with another family when she applied.
I am not sure whether it is too early for you to apply. It may give you lots of time to select, but it also means greater risk the aupair may change her mind. They are young girls, and is to be expected, especially if it is their first time abroad or if they have boyfriends and other commitments at home or just not sure what they want to do.
Are you looking for a summer aupair?
All I would say is that without buying membership, it is almost impossible to take an active part in following up on those aupairs you are really interested, as opposed to the ones who take the initiative to email out rather than just put you on their hotlist. So it is worth buying, once you are serious about looking. I find it a very intense affair once I embark on a search - almost like courtship, but with a few candidates concurrently! If you are in London and you write a reasonably attractive profile, my guess is that you will find someone within a month.