Photo shows two flexible pipes with push-on couplers to 22mm copper pipes. Noticing the bottom cap of the far one was loose, I unthinkingly unscrewed it upon which it slid down the pipe and won't come up again - perhaps it also has a toothed collar? The top part seems secure and completely dry but can I trust it to remain so? Is there any way of raising the lower cap again?
You are lucky if by now if the joint has not separated with the obviously disastrous consequences!!
You should turn the water off NOW and repair the coupling because the cap you have pushed down the pipe contains the ring which holds the coupling attached to the pipe. That's why it won't pull back up!
I can't quite see because the image is small, but some fittings are single use only so the cap cannot be pulled back up the pipe. Others can but it is a little difficult to explain in words.
What ever you do, do not leave this job undone. There is a significant flood risk because the only thing holding the pipes together at the moment is a bit of limescale! In fact, go and do it now!
Thank you plumbob, I called in at Allbits,our very helpful local supplier, who demonstrated how the cap can slide while the projecting end of its inner (toothed) collar is pushed in. It can be difficult keeping on top of this stuff when one only does a plumbing task about once every 30 years.