When touring I ever carried a couple of little white sachets of CE-LIT in the spares, and given it`s the package dimensions it s was easy to find space for it, though never used it in anger thus can nor recommend it as such.

https://www.bhl.co.uk/ce-lit-sachet-display-card.html

It seems as if you have been pretty thorough in trying to identify the leak, though timing cover and water..hmm..? I suspect you are thinking water pump gasket and nothing related to the timing cover other that the appearance of water or staining..?

Mine is an older +8 though it has a weep hole on the underside of the water pump, which is an indicator as to the effectivness of the water pump shaft seal condition, and may just be expected to weep on occasion, perhaps more so after lay-up..? Though if it begins to leak as opposed to weep, that can show up as water marking on the fan belt and even be thrown up on to the underside of the bonnet while the engine is running.

Perhaps you can squeeze a mirror into the area to have a look for the weep hole...?

Good luck.