The visitor's parking area at Henstridge is rough and stony. It's not too bad if you can keep the power down but that can be difficult as they often want you to park half on the grass. This means that you need a lot of power to get on/off the grass.
It might be worth a phone call to see if they can be a bit more sympathetic these days. I see that their website does warn of FOD on taxiways and I have seen some horrendous examples of stone damage from there.