I did switch off and unplug the router and then it all worked which i shouldn't have to do.
The network is 11n consisting of one modem/ router with my main pc in the same room, a second pc in a remote room and my Samsung S3 smart phone.
I spoke to the ISP yesterday and they shuffled me off to the router maker (Cisco) who then tweaked my channel settings different to the ones the ISP gave me when originally setting up the router.
So far it is consistently working .
It seems having the correct channel is making things work.
I will monitor my useage and contact the ISP if there are further problems.
Thanks for your input.