It has come back. To my astonishment it came with a bill for £50 although no repairs were made to it. To rub salt in the wound, I test fired it using Bisley Practice pellets as recommended by the armourer and, far from shooting perfectly, at the 13th round it again started misfiring as before. It is an indexing issue, which is a manufacturing fault.
I am still in communication with John Rothery about it and, who knows, they may restore my now somewhat tattered confidence in them.