My host has advised that mod_security isn't actually enabled at all on the server.
He also checked the site / hosting config, and the hosting is configured with twice the standard site memory allocation at 512MB, so he doesn't think memory would be an issue. He also checked the error logs, but the only thing which seems related is the following as a permissions issue of some kind, Error 403;
POST /forum/admin/?app=core&module=customization&controller=themes&id=1&do=saveTemplate HTTP/2
He wondered if that correlated with the page I'm trying to save? He also said "the file system permissions seem correct, but then this isn't a WordPress site so the forum site may have different permission requirements at the file system level..."
I'm unsure what to make of all of that. Definately outside my knowledge/experience. Any further thoughts most welcome.