Thank you very much for your advice. I have tried the settings on the Snom 360 which did not appear to effect the problem. So, as you suggested, I went through my set up again but I cannot see anything I have left out. A couple of extra details which may be affecting the situation:
I have set the Snom One Soho as the DHCP server.
On my 'mini' LAN there is no router. Only my laptop, the Snom One Soho and the Snom Phones.