or will start with who ever told you the glow plugs dont matter should be shot with a gas powered spud gun for twenty minutes or until they have learnt their lesson not to tell people that.. so what happens is this glow pugs fail and because of this two things are forced in to play one is the car ecu will never regen the particulate filter ie dpf in the exhaust so this will over time partially first block then totally block up, this will cause fault codes for filter for egr valve for boost and even maff sensor and map sensor faults depending how blocked and what car was doing when it decided a enough is enough. the second thing is on start up and on first running car for a few miles the glow plug do in fact help with the heat to correctly help to ignite the diesel oil being spray's in each cylinder on power stroke,
also those cars are known as a wet type dpf ie have a bag of pig fat as such called additive or eolys fluid this gets sprayed in to the tank every time you fill up with diesel and must be added on those otherwise the filter gets blocked, hmm i know stupid design stupid idea for the owner fantastic for the car maker with destroyed engines at worse and believe me keep driving it like it it will kill the bearing on the turbo and eat its engine oil and rev at 10k until no engine oil left or engine bearings, NO A IF ITS A WHEN.
so you have a choice to fix it, either replace the dpf filter and refill additive and fix the glow plugs believe me they do snap off then its head off and more hurt,
or what i have done to both my cooper d clubmans and a lot of other cars like it, dpf egr glow plug delete, its true they dont really do much for starting a car ie will still start from cold at -10.
in my signature is black magic tuning ask for john say spoke to me,, also giving it more power is very good transforms them, they have a mobile van that goes all over or have recovery truck to collect if wanted them to sort it,, or its possible to post ecu to them you knock out the dpf and blank egr valve etc they just write the ecu, you would need a diagnostics run after this to delete any codes