Thanks dal5band and DIYasater. Three of the valves were quickly 'fixed' by tightening as you suggested, and two others are suspect, possibly faulty. I'll replace these next time I have to drain down the system.
Thanks again.
I have a Vaillant combi feeding radiators with (older) Westherm and (newer) Drayton TRVs.
A problem that has recently made itself apparent (warmer weather, = heating not required in bedrooms), is that I cannot turn off the individual radiators using the TRV's. Flow direction seems correct...