To be fair, the guy could of recharged the vessel and the core could of started passing.
Things like this happen and maybe im picking you up wrong but you seem to want to find someone to blame.
Calm down. Far more important things to worry about
It should retract all the way back.
I would take a guess that the water venturi beneath the diaphragm in partrially blocked.
If it was fully blocked you would get no movement
Just wire like I said and you wont have to worry about no voltage or not.
The common comes from the boiler so you'll be fine.
Just don't link live to common.
Just make sure it's dead so you don't get a shock when wiring it up!
When you flip the panel down there will be a cover with two screws, located at the bottom when looking straight down.
You will then have a connection that you unplug.
Wire one to the common on the thermostat and the other to calling.
I remember seeing the b and q "expert" advise a couple of guys on how to fit a flue turret who then went on to buy it.
They were struggling to understand what he was saying as their English wasn't very good
Does the hw light come on when the tap is on?
Check to see if the radiators are getting hot when the tap is on?
Could be something as simple as the flow rate is too fast.