Well there's a lesson learned. When fitting the strikeplate on the rebated French doors I had a flash of inspiration and decided to try cutting out the mortice using my CEL sonic cutter. I can't tell you how much easier, quicker and more accurate it was than the usual method of marking the cut, drilling holes and chiselling out or otherwise removing the excess material. For anyone who's never seen/used one, these things are terrific - the cutting blade is flat and oscillates slightly at a very high frequency which means it's very controllable and accurate (no kick back), there's minimal dust blown around and plunge cuts are very simple to make and keep square/true. I wish I'd thought of using it for fitting the 5 lever sashlock on the opening door but c'est la vie. I'll remember next time and I'm definitely using it for cutting the letterplate opening...