Thanks for all the feedback guys. Given there's currently very little difference between 3rd Party prices and Airline prices (well, not enough to take the risk anyway), I think I'll just go with the airline. I'm looking to book a direct flight whilst avoiding the likes of United and Delta airlines.
Get this though... a direct flight from Heathrow to Fort Worth is about £700 currently. However, an indirect 1 stop flight from Newcastle to Fort Worth - bearing in mind that the stop off is Heathrow, is £550. THAT is messed up and is apparently all down to airport taxes. Pretty scandalous I reckon.