I made a tool to dissemble the cylinder and after work nipped around with the bits to a local hydraulics supplier I'd found in the Web.
They looked, pronounced all looked normal and reckoned was just the seal. Which they found was a less-regular but standard size. So ordered one and bought circlip pliers knowing these would be needed.
Collected seal next day hidden by their door post their closing time. Day after, fitted new seal, reassembled the cylinder - and it was perfect.
"Persona very grata" - not only cylinder good, but ability to repair cylinders as-and-when.
Asked on here and got a recommendation for a book on hydraulics, bought it and read through it that Christmas. Got my own hydraulic pump and cylinder for tests. Worked at a major engineering Co. with a big hydraulics dept. and got to talk with them and look at major hydraulic motors, etc.
The learning path. In context. It is the way for the fast folk on a mission.
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I was able to help you because I had been down that same path. The second-hand gear pump and cylinders I bought for my homebrew tractor bucket loader were worn out, the pump worse than the cylinders. Pulling it apart revealed wear I could remove by surface-grinding the gears and their chamber plates, and worn bronze bushings that I made an expanding internal puller to remove. I didn't fully restore it but it taught me enough to be able to order a properly sized new pump on-line, for a quarter of what local heavy equipment dealers charge.
After prettying it up the bushing puller became my demo to prove I could do machinist's as well as electronic work, and it helped get me the job at Segway.
I referred you to the Vickers manual based on reading recommendations, and it and you being British with your unique threads. My schoolbook was the catalog of the Parker Hannifin hydraulics company plus a small handout to aid beginning repairmen. Later on this became very useful by showing the individual components well enough to understand how they fit together.