In article , Andy Wade spambucket
@ajwade.clara.co.uk writes

Danfoss, IMHO.

I have those. Must be 15-20 years old.

Have never known them leak. Have known them stick shut, over summer in a
badly sludged system,

One or two of mine sometimes stick over the summer, but energetically
working them open and closed a few times seems to cure it.

but they are easily unstuck without the need to drain
down and with no more than a few cc of water spilt.

Any chance of more info?