I’m running Brighton as well.
Have a mooch around Runnersworld http://www.runnersworld.co.uk/general/big-marathon-index/7355.html
I’ve used their heart-rate and pace-based Garmin plans for the last two marathons I’ve done, to reasonable success.
This time I’ve using a Smartcoach plan from the American Runnersworld site http://smartcoach.runnersworld.com/smartcoach. Just able to tailor it a little better to fit my available time, and what I know I can do based off recent race results.
In general, each week you need to be doing one long run at a slow pace, a couple of easyish runs to get some mileage in, and a speed workout. That sort of pattern with the long run gradually increasing towards 20 miles over the training period, and weekly mileage increasing by about 10% each week, should see you right. Also try and get in at least three 20 mile runs before the race and one around 22. Not so much for any sort of fitness, but to teach you how your body will react at that point in the race when you’re running on empty. Those run teach you how to eat and pace yourself, and in a marathon, that’s as important as being able to run at X pace.
Good luck.