Radiator Valves 15mm or 1/2 inch

This is a silly question to all those that know the answer...

I'm changing the TRV's and lockshield valves on my rads and moving the roomstat as per a previous post.

Been to the shed for the lockshields today and had the choice of 15mm at £2.18 or 15mm 1/2 inch at nearly a fiver.

I bought the cheaper one and the tail fits my rads but as they're pre-ancient and possibly around 30 years old I wonder if they need the 1/2 inch version.

Is this an interchangeable unit between metric and imperial?

Further to this I've got 15mm Drayton TRV 4's, should they be a 1/2 inch version too?

Sorry for a dumb moment question but need to get on with hanging these rads.

The copper pipe could be 15mm or 1/2". If the compression ring that comes with the valve fits over the pipe, then it is 15mm. you should then be able to fit the valve with out a problem.

sorry, i forgot. 1/2" compression rings are available from plumbers merchants, Plumb Center etc...

I've just tried the new olives on the existing copper pipes and a piece of new pipe and they fit on both pipes the same.

A further point would be that as the old pipes are very slightly waisted where the previous compression joint was will this seal OK with new olives and fittings?