2019 MK4 ford focus 1.5 diesel.

Exhaust obviously blowing, but reasonably subtle, sounds from front, now according to garage DPf filter jiggered (engine light on, no acceleration in 5th and 6th) they "blew out" , said could see it was degrading, before that regen and  air filter airflow sensor changed (180 quid)about to fit 900 quid replacement, they never mentioned blowing. Don't think they know it's blowing, whole car underneath covered with covers. Could blowing exhaust jigger dpf filter? Has 140000 miles on it 2018 ex works car full history. Engine sounds perfect, drives perfect. What to say to them? Not convinced dpf jiggered or reason (you don't drive it enough at speed) it is.???

Not sure I understand the question here.  

If the exhaust is blowing around the DPF then that will through the pressure and temp readings off.  That may mean it doesn't regen properly or that regen doesn't get stopped if it passes the allowable heat level.  If it gets too hot for too long the ceramic core can go brittle and either crack or collapse.

140k doesn't sound bad for a DPF to need replacement though.  It would have been filled with ash over the next few thousand miles anyway.