According to the linked documentation, LeadPages automatically supports SSL for domains CNAMEd to them. (In all likelihood they use Let’s Encrypt for this, but they don’t seem to say so explicitly.)
So @cliveagate if you follow(ed) the instructions at https://support.leadpages.net/hc/en-us/articles/218820517-Publishing-Your-Leadpage-to-a-Custom-URL and set up a subdomain specifically for LeadPages, or you set up your main domain to only be hosted by LeadPages, you’ll get HTTPS automatically without doing anything.
They also support hosting pages on your main domain when it is hosted elsewhere. So if you use(d) one of the top two methods listed at https://support.leadpages.net/hc/en-us/articles/217777528-Can-I-Use-My-Own-Personal-Domain-for-My-Leadpages- (uploading HTML manually or the Word press connector) and not the above link (which is also listed as a third option on this page), then your main domain must have SSL and LeadPages isn’t involved with that since they don’t host your main domain.