Thank you for those replies.
Visas extended successfully today.
Here's what happened.
For ease of reading in one post I'll state what we supplied for proof of marriage originally and what instructions were given when that was rejected.
Original English language marriage certificate presented.
Certified copy of marriage certificate done by consulate in Chiang Mai in 2015.
(Visa extensions successfully done with the above from 2016 to 2023. In all cases Immigration (two other Provinces) was content to take only a photocopy of the certified copy.)
Immigration
On the first visit we were told "need to prove still married".
When we asked how, we were told "letter in English from embassy or consulate".
We asked what kind of letter and could they show us an example.
"Letter in English" and no was the answer AND at no time did the IO point to the certified copy of the marriage certificate provided and say new version of it.
Consulate
Contacted consulate who, after also checking with the embassy, stated that they believed a new certified copy is what Immigration wanted.
This was their experience with Immigration in Chiang Mai.
Couples were getting a new certified copy EVERY year for their annual extension.
We live in the south so flew to Chiang Mai for the new certified copy.
The consulate advised that next year we may be able to get a new certified copy from the embassy without visiting in person.
This would likely require a local notary to confirm our identities, perhaps more.
Immigration
Extension done and advised a new certified copy will be required for the next extension.
They took the original certified copy and a photocopy.
The IO we dealt with on the first visit was not on duty.
About a week prior to this Immigration Police officers visited our home and took photos mirroring those we had provided for the extension and also of our passports.