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Thanks Ethel, your advice is good - I did that yesterday and since then have not lost connectivity.
The speed has been 54Mbps and has just slipped down to 48Mbps since I started writing this message - could this be because my housemate is also connected?
You're right, my laptop has a wireless symbol on the F5 key, so F5+Fn together work as a wireless switch. However, you can configure this so that it either brings up a menu or it switches wireless on or off. The default is to bring up the menu, which prevents you from switching it off accidentally.
I do wonder, as we're all told that leaving a TV on standby uses X amount of electricity, how much it costs to leave a modem and router on. Since you need to wait a minute for the modem and again for the router is not really practical to switch them off each time.