I know virtually nothing about gas turbines, but I do know that the Vipers in the Shackleton MR.3 ran on Avgas because it was carried for the Griffons.
I "may" have read that they could only run on it for a limited time, possibly because of jet pipe temperatures. On the other hand, that could be complete gibberish.
St Erryn sounds quite feasible as a minor adjustment of RNAS St Merryn, near Padstow.