Thanks for replying,

So I just need to add ServerAlias www.example.com to mydomain.co.uk-ssl.conf and mydomain.co.uk.conf files?

Also is this the best way to issue new certs from Let’s Encypt:

https://www.digitalocean.com/community/tutorials/how-to-secure-apache-with-let-s-encrypt-on-ubuntu-16-04

Or is there a better/easier way?

I followed these instructions, and although its still not working, I think I am close

"
you need to add www version URL on ghost conf file

Open sudo nano /var/www/ghost/system/files/example.com.conf (Open it via Terminal/putty or FTP)

Find this line server_name example.com; Replace it with server_name example.com www.example.com;

If you Install the SSL for your ghost blog you need update URL on SSL conf file too

sudo nano /var/www/ghost/system/files/example.com-ssl.conf

ghost Nginx conf File Location /var/www/ghost/system/files

PS: Replace example.com with your Ghost blog URL

After all set up restart ghost and Nginx

ghost restart

sudo service nginx restart

That's all done :)
"

I found these instructions on Ghost documentation and it worked. Seems a bit more clunky with more files but it uses their script to easily configure the letsEncrypt for the new domain and then redirect it.

https://docs.ghost.org/docs/cli-knowledge-base

SSL for additional domains
You may wish to have multiple domains that redirect to your blog, e.g. to have an extra TLD or to support www. domains. Ghost itself can only ever have one domain pointed at it. This is intentional for SEO purposes, however you can always redirect extra domains to your Ghost install using nginx.

If you want to redirect an HTTPS domain, you must have a certificate for it. This is where letsencrypt comes in particularly handy, because who wants to pay for SSL for a redirect?!

If you want to use Ghost-CLI to generate an extra SSL setup, you can do this using a little trick, first run ghost config to change the domain. This will not restart Ghost, so the change won't be reflected.

ghost config url https://my-second-domain.com

Next, get Ghost-CLI to generate an SSL setup for you:

ghost setup nginx ssl

You've now got two domains setup with SSL. Next, change your Ghost config back before you forget.

ghost config url https://my-canonical-domain.com

Finally, you'll need to edit the nginx config files for your second domain to redirect to your canonical domain. Edit both /system/files/my-second-domain.com.conf and /system/files/my-second-domain.com-ssl.conf, making two changes:

remove the first location block (leave the .well-known location block - this is used for renewing certificates.

for the redirect, you'll need to add the following as the last line of the final block:

return 301 https://my-canonical-domain.com$request_uri;

Once you have made those changes run sudo nginx -t to get nginx to verify your config, and sudo nginx -s reload to reload nginx and pick up your new configuration.