TRV replacement.

Chickereeeee

I swapped my TRV by draining the radiator and freezing the tail (pressurised system).  No water spill at all. I think I had about 15 minutes before the pipe thawed.

Eldi_Dos

WIAWSNB said:

With a sealed system, there is surprisingly little water loss. 
The boiler itself is the part which 'stores' the pressure in an EV. So usually it's boiler off, CH isolating valves shut off underneath it (they might not bother), bleed a rad until no water comes out, slacken valve and drain rad, and then quick swap with only a wee further glug coming out the tail. 
This is because air needs to get in that pipe end, for water to come out. The gentlest of finger presses over the end will stop the air/water exchange.
A tray and towels should capture any dribble.
I wonder if plumbers have their own particular technique?! Please report back with how they do this!

The plumber called round late this afternoon, switched off water at stopcock and replaced kitchen tap, then did TRV as above with the locking of the lockshield whilst carrying out the work.
Said he did not think inhibitor needs redone but will pop in after New Year and check.

Many thanks to all who replied.