The item which I ordered from Argos a couple of days ago, arrived this morning. Its an axial flow, spindle along the length of the water flow, quite well made and very sensitive to the slightest flow through it. You can blow through it and have it count up. It has two counters in the display, a master total and shorter term one. The short term can be reset leaving the master value in place. Both values can be reset.
Both values are shown to 1/10th of a litre or 1/10th of a gallon.
It runs on a replaceable button cell, which is located under a removable coin opened cover, O ring sealed from moisture. This is in the very bottom of the unit, opposite side to the display and the far side of the impeller/ water passage, with connections leading up to the display and electronics.
The display and electronics seems to be completely and permanently sealed in place, in its compartment :-(, so the pipes have to be adapted to reach where you want to read the unit.
My guess is that the Hall Effect sensor is mounted in the electronics compartment, on the PCB, with a tiny magnet on the impeller.
The water input is 3/4 BSP female, output 3/4 BSP male, so it will fit straight onto a normal UK garden tap. It is marked with a direction of flow, but I doubt the flow direction makes any difference - it counts fine with the flow either way.
The unit powers down if no button is pressed and no water flows for 20 minutes and re-awakes on either.
As said, it is extremely sensitive to the slightest flow and its accuracy is more accurate than I can check it. Maximum variation in reading was plus or minus 0.6L in measuring 50L, even with a stop start flow through it, so more than accurate enough for my purpose.
No doubt due to the axial flow, unlike some it doesn't insist on any particular orientation of the unit.
I'm waiting now for 3/4 BSB to 12mm (both male and female) adaptors, to install in the caravan.