tldr; I messed up the transfer of messages to my new phone. However, it worked out fine.
The mistake I made; that the backup on google was up to date. It was a day old. What I'd forgotten that I'd done a lot of messaging that day. So I'd moved the SIM to the new phone, started whatsapp and accepted a restore from google. This was a few hours out of date.
Googling suggested lots of ways to now move the data including 'tough, too late'
Logically, all the messages were still on the old phone. Leaving the sim in the new phone, I removed whatsapp (didnt keep old data), then re-installed, but didn't open it. All this lost was a local database. There is still a backup on google. On the old phone I opened whatsapp and set it up with the old phone number. Then I followed the instructions here https://faq.whatsapp.com/209942271778103/?cms_platform=android which basically tell you to goto settings/chats/transfer chats ie follow the instructions for a normal transfer (which you should have done originally, muppet) You are guided from there, but in short, once the old phone is setup, open WA in new phone and accept the transfer from the old phone, scan a QR code shown on the new phone.
What didnt work. On the old phone, registered to my number (no sim) do a back up. It was over 24 hours since the last one, and it said it was working, but the backup date never changed. When restoring to the new phone, I was missing messages.