Hi Robbie.
Not quite North East, but Allerthorpe Common near York is supposedly a good spot for adders. Never been myself. I only found out about the site through BBC Wildlife magazine, they did a section on adders in March and Allerthorpe was listed as being in top ten places to see them. It was the site furthest North.
Here is a link to Allerthorpe Common website:
Allerthorpe Common
O.S. Landranger 105/106, Grid Ref: SE 761475
Hope this helps.
Paul