I initially thought the bin lorry was a red herring, but with the information that's come out since then it could be the key, and we just don't know where the door is that it opens.
So to continue from my last post. Corrie's in the military, he might think of directions differently than most of us do. He might have a compass app in that phone of his, or he might carry one as standard. If we look at the Barton Mills roundabout we'd need to head almost exactly due east to reach Honington, but it would be over fields. It's a silly, crazy idea, but if he's already had a mystery tour on a bin lorry and this is the way his mind works.. we start off heading through trees, then we reach the River Lark which dissects the route, and I don't know if someone could slip into it in the dark but if they could...
ETA the google map link for the roundabout and river due east. If you zoom out you can follow due east and clearly see Honington on that line
https://www.google.co.uk/maps/@52.3380618,0.5382981,472m/data=!3m1!1e3