Go to the Services at the junction of the A1 and the A605. The lorry and caravan parking area is a long thin bit to the west of the rest of the site. Go right to the end, in between the two rows of lorries and caravans. At the end on the left in the corner is a little plantation of trees, including some oaks. When nobody's looking nip into it and head south about five yards and find the break in the fence. Just though there is an old abandoned road. Everything is very overgrown and the area is totally hidden. Walk a short distance along this road (which is barely visible because of the plant growth) to the east and you will find the best most luxuriant set of blackberry bushes I've ever seen. I picked some but was short of time. They are absolutely at their best; pest free and sweet.
Bill