Well that appears to have worked, so I'll carry on...
I used 12mm MDF for the baseboard, supported by a simple framework of 4x1s standing on legs made from 2x2s.
I produced loads of designs of all shapes and sizes, some using different radii curves, but I settled on a simple design that fits neatly into the corner of the garage and uses just standard Carrera pieces.
The length of the main straight is 8 pieces, or approx 9 feet. The table is 14 feet long by about 11 feet wide.
I've left a crawl through space to get to the corner so that I can easily reach all parts of the track for landscaping.
Anyway, some photos.
End of day 1
End of day 2
End of day 3
With the track in place
Next step is to replace the Carrera borders with my own, make the elevation changes permanent and paint the track. After all that, I'll get to landscaping.
Thanks to all those who have shown their tracks - it's been so useful seeing what everyone else has done.