Just a wee update for folks interested in the diagnostics of this problem. I thought I pretty much had it sorted, and I was on the right lines, its only been some time and combinations of loads, that revealed I hadnt sorted it … and something else, thats let me get to the bottom of it. The EBS system is pressure controlled, so I headed for the sensor on the tag axle air bags as the fault which was causing ‘that’ axles brakes to come on too heavy, when in actual fact, late one night in bed 
as you do … I thought it might be the sensor on the drive axle that was wrong/faulty . The tag drops automatically when drive axle air pressure is exceeded, and I couldnt remember the last time I’d seen this happen. Usually I would lower it manually when I knew I was loading something too heavy for the platform on 2 axles, so, did its thinking for it. So I replaced the sensor on the drive axle too, and it seems normal service has now been resumed, it now behaves as it should, and all hubs are running at similar temperatures at their respective loads. So clearly the sensor on the drive axle is the one that controls the EBS loading between the two. The other item thats been cured is the skipping I was getting. The Scanny OBD kept popping up an ABS fault, which I thought was rust flakes on the ABS sensors on the same axle, so I took them out periodically and cleaned them and the fault went away, however it started re appearing with greater regularity … the fault code revealed which corner was at fault, again the tag but on the NS, and was investigated as early bearing failure allowing the hub to move fractionally laterally on the axle meaning ABS pick up had become intermittent as the sensor is fitted to the axle back plate. There was no visible damage to the bearings on removal and they were a pig to get off, there was quite a rumble form the axle when rotated though, inevitably it would have locked when loaded in this state whilst braking, and this would have given the skipping effect as described in my earlier posts. Ive checked the other bearings as a matter of course, which were fine and so far everything seems to have returned to normal 
And ofcourse Its quite possible the bearing failure was caused/ excacerbated by the caliper overheating due to the pressure sensor fault.
The simple answer would have of course been, to take the motor into my Scania dealer… but I have been the victim of some thoroughly ■■■■ service from them several times in the past and was reluctant to have them throw parts at it like kids toys until they found the fault Anyway … I hope folks find this update useful …