Hi Peter, I have Hunca Munca No10.
I’m in ipswich myself and keep my boat at Pinmill.
I have done a lot of work on my hydraulic system/boat.
Sounds like you have checked the obvious.
My boat suffered from a rotten hydraulic reservoir on the hydraulic pack.
Luckily I had a tank “in stock” I had the new tank galvanised and powder coated.
The emergency hydraulic pump handle needed replacement. The steel handle had rotted away. It got replaced with a piece of thick wall stainless tube (25 x 20 x 800 long, from memory) with a little bit of machining, it was perfect.
The wiring might be worth checking.
The 12v feed for the pump was linked from the engine (it wasn’t pretty)
I fitted new cables from the domestic battery’s, with its own isolator and fuse.
I use HL32 in my system. Any 32 grade oil would be fine.
Regarding hydraulic oil level - with the keel up, the tank should be 1/3 - 1/2 full? If there’s too much in the it will “spit it out” of the vent when the keel is lowered.
I have to dip the tank on mine, through the filler/ vent to check the level.
Hope this makes sense.
Regards Ben.