I'm going there tomorrow but it doesn't appear in the country list. In fact, all the countries north and east of France are hiding. It says Belgium etc on the disc and the missing countries are shown within the yellow line on the map. Am I being really stupid and missing something obvious?

Have you selected the country of choice on the screen

That's the problem. It's not on the list. France, Spain, Portugal and Ireland are there, but no Belgium, Germany, Italy, etc.

Aren't they on another disc? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

So you've got the disc but it doesn't state Belgium
In that case your missing a disc, some came with two discs, some had three discs, but you should have Belgium, whatever set you have.

you don't need Belgium - it's tiny! every road sign lists every town so you'll be fine mate!!!

Thanks for the suggestions. I solved it at the last minute by replacing the 2012 DVD with the 2010 version and got there and back safely. A bit of a miracle given how awful the standard of driving is in Belgium, with roads to match.