When you buy your next new diesel car, ask the dealer where the 'diesel additive' tank is, how much it holds, how much per litre it costs to refill and how often will it need to be refilled.
I don't buy new cars (depreciation is far more than the running costs) but the additive tank is very much adjacent to the fuel tank, it holds only three litres (used to about 4.5l in older models) and needs refilling about every 80k miles. Not in the user manual as it requires resetting of the EMS, so a service item. And it won't last anywhere near as long if the fuel tank is continually topped up with small amounts.of fuel (not many know that, either!).
What annoyed me was the message of 'engine management fault' so take for diagnosis ASAP, without any specific warning. Another 400 miles would have affected nothing. That, and the smaller tank (refilled more often), smacks of dealership profits being raised by the car manufacturer. The previous model had a simple low level indication, so presumably no need for any computer intervention for resetting the EMS.
Oh, and the additive can be bought for about £16/litre.