3rd in the trilogy; at least I hope it's a trilogy!
Having survived Switzerland, the action moves back to England and Templecombe in Somerset before ending up, almost inevitably, at Roslyn Chapel and other nearby Templar hotspots! Including, unexpectedly, Gilmerton Cove. Very much doubt there's an undiscovered tunnel running between the two though! At the very least, apart from all the coal mining in the area, construction work for the A720 Edinburgh Bypass would have cut right through it!

I think it would be awkward to continue the story, although there's no particular reason they'll live happily ever after - whoever takes charge of the Dominicans may still want revenge...

The reveal almost at the end wasn't a surprise - obvious clues were dropped early on and it was more a case of waiting to see how late the 'twist' about Mallory's heritage would be held back.