I fitted the mixer close to the calorifier. I took a T off the cold feed to the calorifier and routed the calorifier hot output to the valve and the valve output to the taps.
It can be a bit of a fiddle depending how much room you’ve got to work in. I’ve attached a photo of my finished installation. Not the neatest, but it works fine!
The push fit plastic tube and connectors work well; however, the connection from the calorifier to the valve should, ideally, be copper as it is taking the full temperature of the hot water.