yep - 6 months up front - I've just done that with a new tenant - situation is - couple just moving in, his ex-partner ran up credit bills and left them with him - he's paying them off under an agreed payment scheme - but that makes his credit score low
I was just going to mention this, you can get around the guarantor requirement by paying 6 months+ upfront. Obviously given the situation you'd have to loan this to her and work out a repayment plan with her, but it would get round you having your name associated to the flat.
My old house mate just moved to London and had to pay 6 months rent up front - 1 bed flat in Camden, total required to move in £14k upfront....