Hi, I have managed to sort of fix this....
I think my problem was that I had disabled the option for the iPads to remove the Meraki Management profile. This appeared to have the side-effect of removing the ability to update it as well.
To get around this,
First I went back to the old certificate - luckily I had saved it & the old hard drive it was stored on!
I set up a new profile with the removable option set to Yes.
In the dashboard for each device I chose to unenroll device for each iPad (glad I only had 43 to worry about)
Then I pushed out the certificate again
Finally I visited each iPad to enroll them again.
So a pain in the neck but not as bad as rebuilding each iPad.
If anyone has any simpler method for next year, I would be grateful.