This seemed to work very well, but for reasons I can't fathom, it stopped my Nextcloud setup dead. And it also stopped webmail.
After much fruitless faffing about, I restored the /etc/apache2/ folder from the backup and nextcloud is cloudy again.
The primary reason to use PHP7 was to keep Nextcloud happy, so this is not a great outcome!
The next day, Symbiosis Stretch arrived, so I upgraded, to find more or less the same problem. A bit more faffing about, then I discovered the cause.
Nextcloud was using Apcu memory caching, and it wasn't installed by default on Stretch's php7 or even installed on an upgrade from Jessie.
Installing Acpu and restarting Apache sorted the Nextcloud issues out nicely.
So far, the Stretch update seems to have gone OK, but this is only on a dev server. Nothing important on it!