TRVs

You can do either.

Provided you can identify the exact make & then manage to buy new heads, then you could change those without draining down.

However, very often the problem is that the pin in the valve body sticks & changing the head wouldn't sort that.

Draining down isn't actually as bad as it might seem.

Well, I already found out how to drain it down when I managed to put a screw through a pipe while sorting out a floor (!) so, yes, it's not too bad. From what you say though, I guess they're not interchangeable between makes (these are all Honeywells), which would make the quick fix less reliable.

Most of the Honeywell TRVs have removable innards that can be replaced without draining by using a special tool. I've no idea what they might cost but might be worth investigating if you have lots of TRVs to replace.

Most of the Honeywell TRVs have removable innards that can be replaced without draining by using a special tool. I've no idea what they might cost but might be worth investigating if you have lots of TRVs to replace.

It looks like I need to spend some quality time with them then, and get to know them properly!

Also, to get them to work properly, you need to balance the system with all the TRVs fully open - preferably with the heads removed. This ensures that the whole house will heat up evenly before the TRVs start to operate. Otherwise you might have to wait for some rooms to get hot and for their TRVs to shut before getting any heat to other rooms.

Sadly, it's more usual for the spring loaded water valve to fail, rather than the head. You can check it quite easily - remove the head then press down the valve operating rod with a finger against spring pressure. Release the pressure and it should return to the rest position.

It's quite common for the pins to stick, though - rather than for the spring to fail. You can usually free a stuck pin either by gently tapping the side of the valve with a spanner or by (again gently) moving the pin up and down with a pair of pliers - taking care not to bend it.