Hi hippytat. Don't know of any green lanes, but I'm from the same area. There are a couple of members at least in the Peterborough area who may have some info.
If you're happy with the car in general, then a replacement (diesel!) engine is the way forward as advised above. Would either of your mechanic friends be up for that?
That made me laugh. The one he tested is about the same age as mine. I pretty much agree with everything he says, but I absolutely love pottering along at 55mph and the sharp edges and having to wind the window down so I can turn right and the torque...oh yes, the torque!