I've just been having a nose at a few websites and found this on netflights.com, I wasn't aware of this myself, and will try it out next time I book.

Flights price match guarantee - terms and conditions
If you find an economy class schedule Carriers* airfare that is available to the UK Public ** from another high street travel agent, on-line travel agent or travel call centre to any destination then we will match the fare.

Before booking the airfare you must obtain a printed confirmation of the competitor's airfare on headed paper or take a screenshot of the web page. This must include air fare price, travel dates, airline and number of passengers.

Please e-mail us at pricematch@netflights.com with the details for us to match the quote. We will then validate the quote with our competitor and make the booking for you.

Price match guarantee can only be applied to airlines that Netflights.com can sell. Price match guarantee does not include any bonding or Airline failure protections charges.

* Applies to UK departing Schedule carriers only (and not low cost or charter airlines). ** Applies to fares any UK Citizen can access and not Student, Senior Citizen or close user group fares.