Ok, I've got the car unstuck. Sur has hotels and restaurants - including a Pizza Hut apparently. If you like a bit of comfort, maybe worth considering as a base if you have a few days. Assuming you can find your way to Muscat , you turn off the highway when you see a sign for Nizwa - at the airport junction or just after. After Bidbid , watch for a sign to Sur and go left at that junction.
Road becomes gravel shortly after.
Between the Al Ashkara and the Al Jafn shipwreck you can turn of the road and drive onto the beach at many places. All beach breaks, hunt around. No crowds - we were the only two surfing on that stretch of coast. But the headland in the distance is where the reefs are. Ah well, at least I brought a surf magazine to read.