Think we’re good now – there’s just a bit of a gap if you’re aged between 12 and 16, vaccinated and live in Scotland, as you can’t use the NHS app.
Called the NHS Covid hotline, after a bit of verification and trying to remember dates and vaccine types, they registered him on the system (pretty sure people aged 16+ were automatically enrolled and issued with a user ID by letter), gave us his user ID which let us complete the registration and download the certificate PDF, which I’ve now uploaded to Eurotunnel.
Pretty much guaranteed now that nobody will check anything, but I’ll report back.
Cheers all for the info and kind offers!