I love non-American English speakers for expressions like "cheeky sod."
A thought on Ennis' state of arousal: not that it has ever been mentioned anywhere by any of the writers BUT, it seems like they've been asleep for awhile and they had been drinking so it's possible that Ennis had one of those early morning full-bladder erections in the middle of the night. So maybe it wouldn't take too much for the clear slick to be produced in an already erect penis. Just looking for answers.
I also like the suggestion of merely being in the tent together as being enough to get aroused, even if asleep. It was obviously enough for Jack.
This sort of reminds me of a tradition in comic book letters pages wherein if a reader sees an obvious mistake in a storyline, they get a special mention if, when mentioning the mistake, they also provide an explanation for it.