Hi, thanks for the replies and suggestions - set up is as follows:-
Firmware 1.01
Stand alone modem - Draytek Vigor 100 - purely a modem providing PPoE to the DIR-645
(I was using a BT HH2 until recently but got fed up with the poor wireless performance and frustrated with the limited settings available so blew the dust of the Vigor and bought the DIR-645)
The DNS IPs are NOT being carried across to the "setup/internet/manual" page but they are, obviously, all in the Status page - I have dynamic IP from BT - unfortunately!
All the other items under "things to try" were in place, as suggested, with the exception of "advanced DNS services" which is not available.
Wireless modes N & G mixed
Security WPA-Personal - auto
Wireless devices - My wifes PC, Android Tablet and Android smartphone - normally none of these would be operating when the WiFi schedule should turn back on at 7.00am - I usually turn the WiFi OFF on the portable devices when they are in standby - saves the battery life!
We have a Panasonic cordless phone system - the base unit is downstairs and well away from the DIR-645 (which is upstairs in my office)
I have turned off the "Short GI" setting.
SO I will try it with the slightly modified settings and see what happens, thanks.