Yeap, where the tunnel starts, just cross over the road , there is a house on other side of the road on the left about 10 meters down. There is a little footpath about 6 meters in length.. That will spit you out onto the dual carriage way.
You can see some steps on other side of carriage way, aim for them. Cross that field..
You are then onto a road junction, go straight over. Follow road to t junction, go left, then 1st right..down that road about 20 meters past canal boat hire place..there is a cutting in the hedge.. You can pick up the canal there and go along to Alvechurch etc..
My description is a bit shite so will dig out a link to my off road commute to Brum.. Stand by..