From "Rules on Drivers’ Hours and Tachographs";
5th paragraph page 6, out of 57 pages;
"Vehicles used for the carriage of goods by road and with a maximum permissible weight (including any trailer or semi-trailer) of over 3.5 tonnes are in scope of the EU rules."
and therefore need a tacho to monitor driving hours.
If your camper carries goods it is a Living Van and requires a tacho and plating. If it doesn't carry goods its a Motor Caravan (on the V5 as such) so no plating, no tacho, Class 4 MOT same as a car regardless of weight.
Indeed, further down that pdf an exception I'd forgotton about from when I did the Class C test is the test vehicle itself, which must be, from;
Minimum test vehicle requirements for buses and lorries : Directgov - Motoring
A rigid goods vehicle with a MAM of at least 12 tonnes, at least eight metres in length and least 2.4 metres in width. The vehicle should have at least eight forward ratios, a closed box cargo compartment at least as wide and as high as the cab"
So the Governments own website describes the test vehicle as a goods vehicle, but the top of page 12 of the first link says a tacho on these vehicles is not required. I'm sure it will have one, because of the vehicles previous life as well as for the instructor to explain its use, but its not needed.
Private HGV is only the taxation class of a Motor Caravan not its use, if it carried goods it would no longer be a Motor Caravan, and your insurance would most likely not be valid.
Crashing while driving tired is of course a "bad thing", but you are no more required to take breaks while on/in your bike/car/Landy than while driving your 8x8 MAN camper.
Totally clear now