Our Drayton TRVs have been installed for around 20 years now and some
don't
seem to be as keen as they were.

I'm happy with my Invensys ones. However, you may find you can get
replacement heads for the Draytons with less tired capsules inside. That
way, you can replace the dry part only in about 15 seconds each without even
needing to drain down, or remove the valve.

Christian.