I would not personally want to trust Nicholsons on mooring spots on a river, there are going to be too many inaccuracies. All this this about having to find a mooring by early afternoon puts me off, as I would not be doing that, so good information seems the counter to that to have the best chance. I looked back at old threads and there are some lists but they are 3 years old.
Unfortunately it looks like you buy 1 day licences or a 15 day licence. For a 50ft NB I make it £33 per day and £109 for 15 days. So the 15 days is not bad on a per day basis if you use the days, but for a shorter period without spending so much I was thinking of 2 days.
I was not sure what "When buying consecutive one day registrations, one of those days can be issued free of charge." on the EA website meant though, when you combine with statement "The term ‘day’ is the 24 hours commencing after midnight and finishing at midnight." The second statement was what I assumed in that 2 days starts a midnight after the time of purchase, hence 3days/2nights it you time it right. But does the second statement imply you get an additional do to this, or are they referring to the same thing, ie first day is "free".