#Edit - This wont mean anything to you unless you've ever tried to fix a Joplin.
I dont know if this will in anyway help anyone but my google searches were futile and seemed to require a special tool. Basically these are DIY instructions to remove / replace the valve assembly in the head / top of Joplin.
After some bodgetastic removal techniques involving BBQ skewers (if I couldnt diy fix this it was for the bin) I realised there was no way it was going to get refitted without some sort of proper tool.
The tool is the one mentioned here in steps 11 and 12 (and later reassembly):
Anyway the holes in the metal seal head assembly are exactly 16mm apart - centre to centre. Hole diameter is a smidge less that the wire in a cheap clothers hanger wire and taper slightly (I have no way of measuring exactly).
With that in mind I found an old one of these flat wood drill bits in the tool box:
So I got the angle grinder and file out, and made of of these. Not neat but works perfectly and solved my problem. Note that the deep V is needed to clear the seal rubber in the middle of the head.
My Joplin was seeping oil from the very top (where the lever presses into the body) and a check in a bucket of water showed its also leaking air (but only when the lever is pulled via remote).
Anyway job done, its working and has stopped leaking air and oil.
Hope that helps someone. Oh and the O ring that failed was a tiny one at the very top of the valve actuator shaft. So small I could hardly measure it but seems common in the cheap o-ring kits I had.
There may be more elegant solutions so feel free to educate me!