The Dolmetsch spinet is the latest in a string of about five instruments that I have converted to play with Delrin plectra. Ist cut the old leather plectrum flush with the front of the jack. 2nd, tap it carefully out from the front of the tongue while supporting the back of the tongue on a block of wood that has a 5mm hole in it.  3rd, cut off .4mm from the top of the old leather plectrum (I'm getting quite good at this!) and replace the leather plectrum in the tougue, leaving a .4mm gap at the top. 4th, insert a Delrin plectrum in the hole, trim at the back and cut to length. 5th, put a spot of cheap 'superglue' on the front and back of the leather plectrum so it seeps round the new Delrin plectrum and holds it tight for final voicing. Superglue does not stick to Delrin, so plectra can still be replaced in the future. They tell me that Morley charges £..... for this. I can do it for £250 per register. Nothing to it!