Good luck, Burnsy and Owain!
Burnsy, it sounds like your preparation is actually pretty decent for a first marathon. I always say it's important to do a few long runs, 18 miles is perfect
For race day, if it's hot, make sure you visit the water stations and aim to drink at least 1 litre of water through the course of the race (preferably more) As Owain said, a couple of energy gels or chocolate bars mid-way through will likely help as well. And try not to start too fast! This is what everyone does in a marathon race. Start >SLOW<, try to keep your pace steady, remember that the last 6 miles are the hard part, and look forward to overtaking some of the people who started too fast later on
Have fun!