TheShadow, thanks, I'll give that a try. I will probably just hook up a Pb battery to the modem DC input socket to try the theory, initially (ie make it independent of mains supply). I am surprised though if this is an issue as it started off fine and has then steadily became poorer with time, since September.
What is odd is how sometimes it takes up to a minute or so to access the router over my LAN. This should be almost instant once the IP addresses are set.