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We have used Lidl Bio washing tablets in the holding tank for nine years!
Works perfectly. Funnily enough the more expensive ones, ie Waitrose are nowhere near as good!
Also our holding tank always looks brand new inside!
The way it works here in Germany if a dealer (or the owner himself) carries out a technical modification to the vehicle, is to submit documentation to a vehicle testing station, such as the TÜV, and if they are satisfied with the modification, they will issue the necessary paperwork with which one goes to the vehicle licensing office where they will amend the details in the equivalent of the V5c. Neither the dealer nor the owner become a second stage manufacturer, but the the modification is nevertheless documented. It's rather like a mini IVA.
However, if the component already has national type approval and the documentation to prove it, the vehicle registration document does not have to be changed, but one does have to carry the paperwork in the vehicle.