"Faringdon" lockshield valves leaking when adjusted.

[Pearly King]

I have just tried to balance my heating system (16 cast iron rads, 3 towel rails), all the lockshields were fully open. When I try to close them down, they all leak, from a dribble to a gush. When I say all, I tried 6 and then stopped. The system was installed 9 years ago. Despite extensive googling, I cannot find anything to say if/ how they can be repaired. The lockshield is not the traditional type, it has an inset screw that is adjusted with an allen key. I am assumming that there is some type of rubber olive that has perished. Can anyone advise if/how I might be able to fix this. The installer has long since retired, so I cannot speak with them.
The attached photos show: the TRV, lockshield, lockshield with cover/cap removed.