Thanks for the comments chaps.It would seem that reliability was a bit patchy. Looking at "Stanwell's" comment in post*2 I remember overhearing one of the engineers, who looks after some of the warbirds at Duxford, saying that the "transport" cylinder heads are the ones to go for on the Merlin. Presumably these are the ones used on the Argonaut and the Tudor.
Somewhat ironically there is a reference in the Eagle book to one of the Yorks taking an Hercules engine to Lisbon for one of the Aquila flying-boats that had gone "tech". Those were the days!