Sounds to me like you either have a duff battery William, (How old is it??, if over four years change it anyway, false economy not to).
But if the battery is OK maybe a bad earth connection against the bodywork/chassis or bad connection from the battery to the starter motor. Check the battery connections are TIGHT and smear with a light coating of vaseline.
Can't see it being anything too serious if it was starting OK a few days ago, but I am NOT really an expert to be honest..Are you in the AA or similar??
Incidentally, for anyone who stores their Moggie for any length of time (over two weeks) I would recommend a good battery conditioner such as an optimate,(around
