short-term flat renting

I'm thinking of visiting London during the week of April 6, but I don't want to impose on anyone I know there for that long. Does anyone know if it would be possible for me to find a short-term flat rental? I know in NYC people who are going on vacation offer their apartments/rooms for rent, even for a short time, because they don't want to pay rent when they aren't going to be there. Where could I look for this? Gumtree? Craigslist? A hotel would seem like the most expensive option, and I'm not keen on hostels anymore.

Check out this site http://www.accommodationlondon.net/

Thank you! While I would prefer not to spend any money on lodging, this won't break the budget like a hotel would, and I can have my own room if I want (which I do!).

I've had much luck finding an apartment for a weekend in NYC through Craigslist and in Paris through VRBO.com. DH and I are going to London for a weekend in late March and are considering using one or the other to find an apartment then.

Heres a site for you

http://www.britainexpress.com/great_british_sites/London-flats.htm

There are plenty more - search under Self Catering holiday flats / appartments London - and google will chuck up quite a few sites for you.