Hi Tom,
Thanks for all the work you put into the reply, it will keep me going for ages. The last time I had anything to do with matrixes was rotating wire frame graphics, also on a BBC B 32k which I bought Christmas 1983, for a months wages I think, from the NAAFI in Bielefeld where I was posted. Prior to that, in school, where we did them endlessly it seemed, and never knowing why.

Gareth