Cheapest flights to quebec/montreal in august?

Where could i go to find the cheapest flights possible to canada for the beginning of august staying out there for four weeks???i will be flying from london
Answer
Http://www.flyzoom.com £549http://www.cheapflights.co.uk/flights/montreal/london-area/ http://travel.kelkoo.co.uk/c-172201-flights.html compares many sources in 1 go; £480 using 3-31 aug.http://www.canadian-affair.com £507 with thos. cookhttp://www.bookingbuddy.com £460 with swiss