Hi Nick, I think the way I see it is:
Same route as the 14 miler + 2 extra 6 mile laps = 26 miles.
Conor - sorry just saw your message - long runs are the staple of marathon running especially your first so get those in, although in reality you only have 2 more long runs left to do 3rd and 10th Feb, mine will be 2 x 22 miles, but you need to do what you can in relation to your training etc. Then 1 x 10 mile on the 17th Feb.
Feed stations at every 3 miles makes it nice and easy so you will not need to carry water at all.
Should be good running in the New Forest, I have done their Downton Half a couple of times and have enjoyed it ...