A quick update; Talktalk support were of no help - they really only offer help to connect one device so there was no enthusiasm to help me connect a third device.
I googled this and it seems that I'm not the only one with this problem and the consensus seems to be that it's a router channel issue. Before I could get into trying different channels the machine got itself into a cycle of booting and restarting from the desktop. How I managed this I have no idea but I've decided to cut my losses and re-install the operating system.
Thanks to all for the suggestions.