PP, what you're confused about is the two types of position reports.
The first type, as part of the "pass your message" message is where you are *at the time you're transmitting*. Eg. 15 miles south of whatever. No need to specify time (yet) but other details are important (e.g. route). If you happened to be put on "standby" when 15 miles south, but only told to pass your message a minute later, you simply report "13 miles south of whatever" instead. You are correct in the sense that, if required, you can put in a next point and an ETA for that point. Eg. when requesting a MATZ transition, it would help if you give the VRP where you'll be entering, and an ETA for that VRP.
The second type is a position report which is prompted by an earlier "report position at xxx". In this case the controller wants to know exactly when you were above that reporting point, and your eta to the next reporting point, for traffic sequencing purposes or as part of its "alerting service" role. But if you happen to be overhead xxx but someone is blocking the frequency, you still need to report when you were overhead. So it includes the time.
BTW I don't know if it's legal, but if the frequency is empty when I'm overhead the position report point, I just report my position "G-CD overhead Ockham expect Dettling 47". I was also once asked to give my ETA for the waypoint after the next. First time I ever told ATC to standby.
Oh, and does CAP413 describe the situation where ATC passes your message to you instead of the other way around? Here's what happened yesterday: I was practicing steep turns, stalls etc in the Rotterdam ATZ (class E) at 3000 feet. In contact with Rotterdam Approach, on a discrete squawk. When I was finished, the following happened:
"Approach, PXX finished playing, returning to Rotterdam via Hotel."
"PXX roger report passing 2000"
"Wilco 2000 PXX"
"PXX passing 2000"
"Roger contact tower 118.2"
"118.2 PXX"
"Rotterdam Tower, PH-XXX" (Just making contact and mentally preparing to pass my message...)
"PXX, Rotterdam Tower. Cleared Hotel arrival 1500 feet report Whiskey. Weather is T, QNH 1008"
...Stunned silence for a few seconds... That's supposed to be *my* line, isn't it?
"Hotel arrival 1500 feet, wilco Whiskey. Information T received, QHN 1008, PXX"
Gotta love these guys and girls...