Radiator lockshield valve - top part has popped out

[UnusualCapybara]

Hi,

The radiators in our new house seem to have been turned off for quite a while at the lockshield valves. I've turned all of them back on with exception of one which was very stiff and I've clearly applied too much encouragement as I'm left holding this piece.

It doesn't look sheared, more like it's popped out but I may be very wrong. Can anyone advise if I'm able to get this back onto the valve somehow, or if I need a plumber to change the whole lockshield valve?

Thanks

 

[tincup]

I would change the lock shield for peace of mind while the weathers mild.

 

[flameport]

That has broken off.
New valve required.