leaking ball isolation valve

Popped in to the in-laws today to collect the pup, to find water dripping through a ceiling.
Traced it to an isolation valve in a bathroom, but waht I didn;t understand was it wasn't the valve to pipe joint that was weeping, but the actual operating arm - the bit with the slot for the screwdriver - that was weeping.
Obviously its get the plumbers in and get it replaced - I am not going to spend hours finding out how to drain someone elses bloody CH system - but the question is, how are these bits normally sealed? I thought the balls were an interference fit in the housings, and water couldn't come out of the pivot.
But the whole ball assembly seemed loose.
As an aside, me and pa-in-law wound some PTFE tape round it, every which way, and wrapped it in PVC electrical tape, and that's stopped the drips till the plumber arrives, so there's another bodge to add to yer repertoires.
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The Natural Philosopher wrote:

Yeah, theres a bit of rubber in there for sealing which is probably u/s. Although I've never seen this problem in the many I've fitted...
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