Hi Hazel.
You are quite right to be looking for a braethable ground sheet. These are the prefered option by sites these days.
We have just bought a woven nylon groundsheet fron towsure. It is approved by the caravan clubs etc. To look at it you would think it is solid, so hopefully we won't have grass growing through like we had with the open weave groundsheets.
We are currently on a gravel hard standing, and I have put a blue plastic tarpauline underour new groundsheet to keep it clean. This is an acceptable practice for hard standings.
Our woven sheet was