Thanks for thhe link, according to that (and also it helped by looking at the inside cover) I have a Sundial Y plan with a V4073(x) valve. So I can test power to it at least and I know a lot more about my set up.
Well not quite hot water actually and tons of it. Turned off power to unit. Detached the unit and as I did the spring sprung back. Re-attached with the lever (being spring loaded now). Turned power on, switch on heating 10mins later hot water.