I can't believe I've been an FT member for this long and I'm asking this question, but if I don't know, I figure that others don't know either.

Friends are moving to London for a year. Given that they are renting a large flat, I'd like to maximize my visits while minimizing my travel costs. What's the best way to find cheap tickets to London when my potential travel dates are, quite literally, any time (with the exception of a few weeks) within the next 12 months? Every site I know (Kayak, etc.) limits my flexible searches to within a couple of days.

ETA: Google's flight search function is pretty close to what I'm looking for.