Thanks to those who replied. I had already called out a telephone engineer who said that he really couldn't find any equipment faults but he changed one of the filters anyway and now it works OK. Coincidence or not I'll have to wait to see I guess. I have since been told it may be to do with the DHCP settings and that I need to make sure it's not trying to conect to a specific IP address and it should 'Allow automatic IP adress assignment'. I'm not toucjung anythingnow though as it's working OK.
Many thanks