I am quite sure that this topic has been raised before, but can any one recommend an establishment anywhere in Australia that can refurbish, clean & test hydro bags? I don't want to screw it up by trying to do anything myself...

I don't think there is much you can do that will "screw up" a displacer if you strip and repaint the canister and clean the hose.
The main problems are with the swaged ends of the hose letting go as the rubber will shrink over time. This is repairable. Internally, they just seem to keep working, which is good as there is no adjustment!
Avoid putting anything into the displacer other than hydro fluid.
I have just fitted a set of displacers that have been stored for over 25 years and they are holding pressure.
Leaving some dregs of old hydro fluid in stored displacers might have helped preserve the metal parts of the internal valving.

Drmini in Australia (Sydney NSW)...but compensate him for his efforts please

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"Show me the Mini!"

@9YaTaH
Yes Mick, I have tested a few for my own use and replaced a couple of hoses, however I am retired now and not running a hydro bag repair service, sorry.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Ahhhhhhhh awright...you try to help pensioners earn a little beer money and they say "not running a hydro bag repair service, sorry!"

Keep on truckin tho Kev

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"Show me the Mini!"

That's a bummer Kev! No worries.