Addblue woes

[colingray]

2015 2.2 diesel has the a warning on the dash 501 miles time engine will not restart, ref incorrect Addblue specification detected.
This has come up on my daughters car and I wondered if anyone has any advice on how to sort it out without emptying the bank account at the dealers?
It seems to be a fairly common issue so I am surprised there is not more content on this forum.
thanks
Colin