I now have the book "Shropshire Airfields" by Toby Neal which does nothing to strengthen the High Ercall case, although it does raise the profile of Sleap in addition to Tilstock WRT Glider Ops & Trg.
Interestingly I noticed yesterday that the ATC tower at Tilstock, neglected for decades, is now covered in scaffolding. Not usually indicative of destruction, so perhaps a second life awaits after all these years.
The one thing High Ercall does have going for it is that frequently bombers unable to land in East Anglia or Yorkshire diverted in due to weather.