Thanks for all you help chaps. The problem did in fact turn out to be caused by a dodgy and dying graphics card. I'm delighted to report that I now have the old girl back with me running a much better Nvidia GTX 970. Couldn't be happier, particularly as I've successfully sidestepped the new iMacs with their pretty well welded-in everything. Also huge thanks to UK based Create.pro and Lewis at Brighton's very own themac-man.co.uk . You Americans are great but it's good to see we have some home-based talent too!