We've just been activated (business account) for Zen ADSL, I see there's an option in the portal to set an rDNS - am I able to set this to something.ourdomain.co.uk without the domain being hosted by Zen?
Yes, but only after you have set up DNS for something.ourdomain.co.uk and after this has propagated successfully and resolves to your new Zen IP address. Zen set up mine[1] in response to an email, before the new portal feature was introduced, but I think the new system will automatically check DNS before setting up rDNS.
This is correct, you must first have an A Record setup in your DNS to point to your Zen IP, then Zen will configure the reverse, the portal does a live check on this (which is where the intiail problem with this module was)