On my recent trip up north I had an Ad Blu fault come up (21 plate Berlingo, 40K miles) took it to a mates garage, diagnosed as injector for Ad Blu pump, said once one bit goes, another does, and by the time you've replaced all the bits it's about £4K.
Can get them remapped for anything up to £400 which removes the need for Ad Blu and is not a factor in a M.O.T.
Cheers, D.