I normally use Kayak.co.uk
You can't book for them, its more like a price comparison site and it searches the airlines direct as well as ebookers and opodo and others.
Once you get your results you can narrow it down by times, airlines, stopovers, where you stopover and price. It even gives you the best prices for the whole month before you search.
I just had a look and it had LHR
for £242 in October
I find it good to use to get an idea of prices as well even if I don't make a booking.
I also like www.alternativeairlines.com
they've normally got one or two good deals but they're not always that good for prices.