Hello Adam,
Thanks for being in touch with the forum.
As I see it, you are questioning your garage’s diagnosis of your issue - and there is nothing wrong with double checking, if you have valid reason to disbelieve such diagnosis. As Cliff says, it would be very difficult for anyone on here to ‘guess’ the possible causes, without access to the car.
If this were mine, I would not be bothering with ‘normal’ garages, but seeking advice from a recommended, trusted, local diesel specialist. As I keep repeating, to a point of boredom, ask around your local taxi rank for recommendation on local diesel specialists, since taxi owners depend on good service at reasonable rates.
Perhaps you could let us know how you get on.
Kind regards,
Gareth.