One site on domain-mapped multisite network broken after adding new site

Hi guys

I have a multisite network that uses Domain Mapping, WP Encrypt and Really Simple SSL...
On the whole it's really easy to add sites and it all works well... All sites have forced ssl etc.

Just now, I added a site (oliverdrake.plumbing) that took some fiddling to get it to work (including deactivating the Really Simple SSL plugin and reactivating it) as it was getting stuck in an infinite redirect loop.

Anyway, that's all sorted now but for some reason, one single site on the network is now broken (connection refused). This is https://laplumbing.co.uk/

The DNS resolves correctly and the ssl checks out too:
https://www.whatsmydns.net/#A/laplumbing.co.uk
https://www.sslshopper.com/ssl-checker.html#hostname=laplumbing.co.uk

The mapping uses https but I have also tried http
I've tried with ssl enabled (in the Really Simple SSL settings) and disabled too.

The primary site for the network is https://madebymartin.co.uk and I have granted support access. If you do log in, please tread carefully as these are all live client sites.

Any help would be massively appreciated.
Thanks in advance
Martin