Thanks for all the replies, far more than I was expecting!
I've ordered a black Edding 8404 on ebay (£3.99 delivered), will come in use even if not for this application. Just hope its genuine.
Regarding the Aldi protractor, I've got one of those but zeroing it on a surface plate doesn't work well because its not dimensionally accurate and there is a small angle between the blades (~0.2 deg.) when "flat". I usually use a 123 block to get a reasonable 90 degrees and work from there; that's if there's any battery left when I take it out, often there isn't.
I've got a fondness for old tools and getting them to work again which is why I'm taking the trouble. I've got ancient M&W, Starrett, Mitutoyo, Grey & Rushton, etc stuff, as well as up to date quality tools.
Charles