Sorry for the delay in posting back - the other side of the wall is my Study, we do have trees in our front garden but these are mainly crab apple trees.

The roots start from about 1inch below the surface and seem to go down about 4 to 5 inches and also grow across as well as down, if I let the roots grow then they end up very long and think (about 10mm thick) + they are not easy to drag out of the ground.

I did however cut down some Ivy a couple of years ago, could it be that?