Thank you all for your kind words and encouragement.
In case it helps anyone building their own C&C, here are:
THINGS WOT I LEARNT WHILST DEALING WITH CORREX.
1. 8ft x 4ft correx sheet is bigger than you think. And 2 bits of it are twice as big as that and will not fit in your living room. Cut to length (plus 300mm or so to allow for error) before you bring it into your working area. Or be sensible and wait til summer and do the whole thing outside.
2. If you need to join correx sheets, do it before you cut to size and shape. That way you can...
3. Make sure any joins will end up in the middle of the finished base and not so near one edge that it all goes wrong when you fold it.
4. Have a new blade in your Stanley knife.
5. Measure things at least 3 more times than you think you need to.
6. Cutting correx is ridiculously easy one minute and stupidly hard the next as the angle of your blade changes. Makes no difference whether you're cutting along or across the flute, you'll be going along speedily and then suddenly come to a going halt.
7. Ultimate duck tape is really, really sticky.
8. Be aware that if you cut your correx using a nylon plastic chopping board to protect the carpet, there
will be a static build up which will discharge itself when you turn the correx over and give you a shock. Try and make sure it's not on your Hoo Ha or Man Bits.
9. Brooklyn Nine Nine is an excellent choice of background TV.
All that said, it wasn't too awful and took around 2 hours. Feeling quite proud of myself