Tranceliner
If there is one thing I know about modem/routers is that their firewalls are not switched off when they say they are via the GUI, I believe modem/router firewalls can only be switched off via the command line, i.e. DOS prompt.
The following link will take you to the port forwarding instructions for the Belkin F5D7632-4v3. Sorry, I could not find the UK model you have but I am reasonably sure that these instructions will work:-
Port Forwarding for the Belkin F5D7632-4
UK Bob