Not a direct answer, but we (I mean the company I work for) supply fuel system cleaner to main dealers, It is used as add on sales at the dealers. It is cheap and nasty and i wouldn't use it. But giving the dealer the benefit of the doubt and assume they are using Forte or a Wynns product (which we also sell), it still says in the book don't use these additives, so its a cheap shot at getting 15 quid tagged onto your bill.
If it was approved wouldn't it be in the VW bottle and available to buy over the parts counter like the screen wash, oil and coolant? Perhpas you could ask for a bottle to take away and see what they say
