Further to my last posting. The tag/system seemed to think i had entered incorrectly, so charged me for the entire stretch of autoroute instead of 6 euros and ignored the fact i had returned to the same auotroute an hour later.
SANEF UK want chapter verse and colour of grandmothers eyes (I exaggerate but not by much) and now tell me the matter has been referred to France but that any credit / refund may take several months. Not at all what I had expected.
So much for the 'easier to have a tag' suggestion. Advice remains to check you invoice as soon as possible and challenge any errors immediately. You will need date and time, Autoroute section used, vehicle type reg and colour, whether you had a roof load, height of caravan, tag number etc.
The implication is clearly that you are at fault.