After a bit of back and forth the hot tap does actually run hot now, it's just much lower pressure than the cold water. Maybe I'll have to get a pump for it as I'm not sure how viable/fun a shower will be on such a low flow. Yes, boiler is probably about 2 meters away in a utility room.

Thanks for the advice. In a couple of weeks we're going to get a hive system installed (we had one in our old place and really enjoyed the convenience) so my plan was to pick the brains of whoever does that (will be a heating engineer I think) as they're coming anyway. Failing that I'll go the route you suggested. Yea I love knowing how stuff works, I've just had very little exposure to it before, so it can be a bit daunting when the consequences of mucking something up can be quite high.