Leaking oil valve, from the top...
Discussion
In theory you can use a spanner to tighten the packing against the shaft. Do NOT over-tighten.
https://www.youtube.com/watch?v=Tb7RYE4KOXk&t=...
https://www.youtube.com/watch?v=Tb7RYE4KOXk&t=...
Is that not a safety valve, if it is you would be better replacing it cost under £10
https://www.bes.co.uk/handwheel-fire-valve-10809/
https://www.bes.co.uk/handwheel-fire-valve-10809/
Edited by mick987 on Sunday 22 December 20:30
alfabeat said:
It is unfortunately dripping badly. They haven't had the aga on yet and thought they would fire it up for Christmas...... Their chap who looks after the Aga is away on holiday. They have turned it off now and it's stopped dripping.
Leave it be then. 
Last thing you need is to try and help, and end up making it leak nonstop.
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