Well, that gland nut has been tightened fully home. Normally there are a few threads visible - are the others all fully home all as in the picture?. This leads me to think that possibly that valve does not have a traditional gland packing and so you won't be able to service it in the way you intended. It should still unscrew, but if it was tightened fully home in the factory, you'll struggle to undo it now.
Of the non-leaking valves you have, is there a definite resistance to rotation when you turn them or do they turn easily? A correctly set up traditional gland will usually feel a bit stiff whereas O ring sealed glands have less resistance to turning.
3 in 1 oil won't dissolve limescale. It was actually (if legend has it right) designed as an oil which would coat the exposed surfaces of a bicycle chain (and so tends to clog precision mechanisms). WD-40 isn't much good as a lubricant, but it's good for cleaning purposes and it does seem to shift limescale, which was why I suggested it.
Thanks very much for your latest reply Ric. I am learning a lot from you!
3 of the others are tightened fully home as in the previous picture. Just my luck! I don't know about the 4th, as it is a different make to the others (will post pics of the caps) and the plumber stopped that one leaking. It doesn't have a screw for me to take the cap off and have a look underneath like the others and I didn't dare force it off, in case it started leaking again.
The 3 radiators with the nuts tightened fully home turn easily and the different one is very stiff since the plumber fixed the leak (you will see in pic, it was so stiff I needed elastic bands to be able to turn it off after the plumbers left).
So, I am guessing from what you say, that one has a traditional gland and the plumber was able to tighten the nut and fix the leak and the other one he tried to fix must have an O ring sealed gland, which is why it is still leaking? He didn't say that though. Said he had tightened the packing nuts and they were no longer leaking. However, when they had gone and I checked them, only the different one (with white cap) was fixed and the other one (with grey cap) was still leaking. Seems to be leaking worse, so maybe he did do something to it.
Today, I tied a shoe lace around the bottom of the spindle, so I can direct the leak into a plastic container, (from some advice much earlier in this thread about tying some string and using natural capillary action). Seems to be working. Might the Fernox sealer fix the leak temporarily, if I put some around the bottom of the spindle?
I have gone back to my original fear of having the plumbers back in, so I might have another winter with all the leaks! I have seen a bigger drip tray for the hot water cylinder that should fit perfectly and need emptying less often. Hardly leaking at all at the moment but as soon as I heat it up and use the hot water, that's when it starts leaking more again.
At least I had the pilot light fixed on the boiler, which I wouldn't be able to do without in the winter. Oh God, I hope I haven't jinxed it saying that!