Sorted it today.
Pure Highway now working in the car without the Pure accessories.

Just for info on the forum. And for DaveDND
The aerial connection uses a 2.5mm (stereo) jack plug. Maplin code FJ85G.
Numbering from the "tip", where the tip is 1.
Connection 1 = +3Vdc (I think this must be for the aerial/antenna signal amplifier, or just to stuff up anyone who tries a non pure aerial

)
Connection 2 (middle one) This is for the aerial input*
Connection 3 = Ground 0V
*Now the middle connection no:2 had me stuffed as it measures open circuit (no resistance).
When I used a DAB mag antenna that was supplied with my Marantz ST7001 I got very little signal level, barely 1 bar.
But on plugging the Highway in the car, with a aftermarket 5Vdc USB Power supply lead, the signal level jumped to full signal (all bars).
I suspect the 2.5mm jack input is disabled on batteries, and is only enabled with an external 5Vdc supply.
I personally would shoot the designer, this is clearly aimed at ensuring you only buy Pure aerials (or the Kinetic one available elsewhere)