You would need to remove the cloudflare proxy from in front of your main site to enable us to add that hostname to the certificate. Right now we cannot renew the certificate at all, due to your cloudflare configuration, which sends web traffic to cloudflare, not to us directly:
$ host tickets.lso.co.uk
tickets.lso.co.uk has address 104.26.5.91
tickets.lso.co.uk has address 104.26.4.91
tickets.lso.co.uk has IPv6 address 2606:4700:20::681a:45b
tickets.lso.co.uk has IPv6 address 2606:4700:20::681a:55b