Anyway, my router/modem played up again this morning so I spoke to Orange. They said they had checked the line and (as expected) all was OK. Had a look on the internet and spoke to a real I.T. geeky anorak type. It's possible it's caused by the router getting overheated. As I had left it turned off on Sunday night and it was OK on Monday morning 'til now maybe that's the answer.
Growler, I know you said you switch off the kit overnight but maybe your router is overheating. It does seem a common problem from what I perused on the web.