Question Author
Thanks folks.
Some other research has revealed an interesting couple of quirks.
Several from TV Chefs: one uses digestives and pecan nuts, with butter and cold set, another uses digestives with cinnamon and cocoa, butter and cold set, another uses digestives and ginger nuts, grinds to a powder before mixing with the butter and baking for 15 minutes, yet another uses digestives, demerara sugar and butter - cold set, whilst another used chocolate brownies - yuk!
So plenty to experiment with as Illya7 suggests!