New radiator valves

It depends whether the stub of pipe is short enough to allow the olive to "seat" - if it does (and cosmetics are not relevant) I would choose option 2 (with a smear of boss white, of course). Option 1 has the risk of nicking the pipe, or the pipe having been slightly compressed.

You can also get olive pullers that may do less damage to the pipe.

You may find that your new valve is a different size to the existing valve, especially if it is very old. You may have to lengthening the existing connecting pipe. I had to do this on my 20 year old system.

That's interesting, but I think this is better:

Depends on the circumstances.

If:

a) The new valve has the same size backnut and pipe fitting as the old, and

b) If the new valves pipe recess is deep enough that the old olive makes good contact with the sealing face, and

c) the result is cosmetically acceptable

Then go with option 2.

Otherwise you might need option 1, or the even more involved option of changing the valve stub in the rad itself.

Older valves used 3/4" BSP nuts, and newer ones tend to use 1/2". Also older stubs often had machines ends designed to mate with a cone fitting on the valve body. New ones are often designed as plain pipe ends, and the valve has compression fittings on the pipe inlet, and the outlet to the rad. The newer style gives a bit more flexibility in valve spacing from the rad.

I should have said that we're also changing all the radiators, too.

Consider increasing the size of the radiators which may/will increase the efficiency of your boiler/system by allowing you to decrease the flow temperature. This assumes a modern condensing boiler or heat pump.

You may find that increasing a type 11 with a type 21 or type 22, or a