In reply to Maircolin:
My wall is about 15 feet high, i.e. the same as a commercial bouldering wall which is high enough to hurt yourself so I just coughed up and bought a proper crash mat from here
https://www.foams4sports.co.uk/product/promat-crash-mats/
which cost me about £250.
My reasoning (as well as not wanting to break my ankle again) was that two of us use the wall and payed £125 each which, including fuel, pays for itself after 13 uses when I would have otherwise driven to a wall.
Annoyingly, about a month after I bought it, the PE dept at my wife's school had several surplus crash mats to get rid of so I could have saved the cash. Still, I've got 2 crash mats now!