Hi Looks like that's been leaking for a while. unfortunately any sealing washer is on the inside of the cylinder. I used to make a point of checking the nuts were tight on a new cylinder, they did need a bit of a tweek on occasions What to do now - Well if I was doing it, I would drain the cylinder, back off the nut, and (being old fashioned) make up a grommet from hemp and joint compound. PTFE could be used, or at least worth a try. This could well be a case of getting some professional help if you're not used to plumbing work, or haven't the tools. But don't be surprised to be told a replacement cylinder is the answer.
Thanks - I do have the tools and some plumbing experience.
Are you saying that because the seal is inside the cylinder then replacing only the fibre washer is not enough? You suggest hemp and "plumbers mate" made into a washer to make a seal on the outside?? Where do I get hemp?
A proper plumber's merchants should be able to supply. But NOT Plumbers Mate, you need an ordinary jointing compound such as Boss Green. I would suggest winding hemp and compound around the thread, but keeping it as close to the cylinder as possible. The intention being to create a grommet that will compress onto the thread at the same time as against the cylinder. That's a lot easier to do than it is to explain ! Best of luck with it, but, as I've said, you may end up with a new cylinder.
Thanks for your help - I have completed the job and it is not leaking - so success! It saved quite a significant amount of work and costs as the cylinder is built into a cupboard which on one side is also a fitted wardrobe. While I needed to remove some internal shelving to access the cylinder bleed valve it saved me removing the entire floor to ceiling frame and doors which would have been necessary to replace the cylinder.