I've sorted this now. After speaking to Vaillant about 7 times, one person there said an engineer would need to put some new links in, another said it hadn't been wired up to manufacturer specification then cut me off (I actually got cut off from them 6 times before getting it sorted). And...
Firstly, I'm not going to attempt to do any rewiring my self - but I do have a Gas Safe plumber on the job currently (waiting for an internal pipe that is leaking a bit), and I want to get informed on our situation so when he comes back we can get it sorted asap.
I have a Y plan heating setup...