Yep, just undo the metal collar, remove remnants and replace either like for like https://www.toolstation.com/drayton-trv4-classic-thermostatic-radiator-valve/p39498 or compatible products
A photo generally helps, but I suspect it’s a rebadged Drayton LP programmer. I think some were replaceable others weren’t. If it is the Drayton and you can’t replace the battery, then a new unit would clip onto existing backplate.
Obviously depending on the actual size, that would seem ok, buy perhaps it's struggling. Maybe change it to 16Kw? Maybe get him to speak to Vaillant technical? If anything it should overheat in chimney sweep.
This might be the case, however these are gas safe engineers installing the wiring. If the electrical manager isn't fussed about them, neither am I. All I do is ensure i have tried to install any wiring i do to a good standard.
I have been guilty of saying that people who are not gas safe should not open certain boiler covers, but a recent re-read states you can if you are installing a control.
No, a lot of the ones im talking about are wireless stats. Dickheads using cpc as a switched live/line. Electrical managers answer was to put brown sleeving on.
No not drive by's and a lot of our sparks are good. It was more if others than my diligent self entered it might have got left. Only people qho may have spotted inside the boiler other than me might be the auditors.