I have precisely the same scenario (mine is 40 miles distant!)
See:
http://www.frequencycast.co.uk/homehub.html
http://www.frequencycast.co.uk/homehub.html - although I noticed when completing this guidance that the Homehub already appears to have a pre-defined forwarding def for 'VNC' - one just needs to apply it and then add the specific device (ie. your Dad's pc) that needs to be forwarded the ports...
Cheers,
Aitch