We didn't even realise it was there until recently. My guess is that
someone thought it would be a great idea to put down weed control
membrane covered with a few inches of bark mulch.

Fast forward a few years, and the weed control membrane is covered in a
few inches of humus rich soil. And the shrubs planted in it have spotted
this nutrient rich later, then sent roots over the top of the membrane
as well as underneath; in places they have found or made holes and the
roots go through the membrane.

I've been unable to tease away the membrane from the roots of the
shrubs, and don't dare disturb the perennial plants that could be in
holes in the membrane - or just rooted on top.

Ah well, it's been great exercise. About 1sq metre per hour.

Andy