Thanks again newdigital for your help. I have done everything possible - re-installed MT4, re-started computer ( in fact it's after several restarts later that the old templates appear).

The data must be stored somewhere in Windows outside of the MT4 program folder. I have done a windows search looking for anything called mt4 or similar, and come up with nothing. It's a mystery, and very annoying. If anyone knows, please tell me.