Graham - thanks for your reply. I have always disconnected everything when I went on holiday - and used to do the same every night - but was advised by my ISP not to disconnect my broadband. I think if there is any danger of storms in the area I will ignore that advice in future!
I think the idea of not connecting overnight was something to do with maintaining the connection. I had trouble connecting one morning and phoned them (Madasafish). They seemed to indicate that my connection reliability and speed would both improve if I left the connection permanently on.