Some websites are not accessible on DGN1000 and few other NetGear Routers. It is something related to router settings and not related to ISP problem. After few hours of research found a solution to solve this router settings problem.
So finally a resolution. MTU seems to have been the issue, though I have not found an explanation why. Note, that this behaviour was found when I tested both netgear routers I have access to: DGN1000 and DG834Gv4, but not my old DG834Gv2.
To recap. If a schedule is in place to limit internet access to certain times (allow by schedule, otherwise block), PCs running WinXP or Ubuntu linux were unable to access certain websites at any time (Hotmail & Yahoo mail). Removing the schedule and refreshing the browser page with no other action being taken allowed the website to load.
Set router MTU to 1458 as recommended by plusnet if there are issues.
Set MTU in linux (simple, but took a while to work out how to make the change permanent) to the same.
Set MTU in windows (the microsoft tool did not seem to work. Dr TCP did though) also to the same.
Subsequently the problem sites have loaded no problem with the schedule in place.