I used to track down codes for members who couldn't track down the correct code for their V+ a few years back and Goodmans TVs were always a pain as they used other manufactures parts so codes would reflect who had supplied the relevant bits.
At the time I was advised that it was not VM's responsibility to track down codes but that it was TV manufacture's responsibility to inform VM, Sky etc. Maybe a staffer could confirm if this has changed?
However as you have tried the full 0000 to 9999 test then I would have to assume VM haven't been given a code for your TV and maybe you would have more success contacting Goodmans direct.
http://www.goodmans.co.uk/support/faqs.html
You can use the Live Chat facility Monday to Friday between 9am and 5pm.
Add/ Grundig is another name associated with Goodmans so you could just double check any codes you can find associated with that brand name.