54 plate, cdti 100k

Noticed front pipe blowing and slight loss of power - garage suggested related. Got pipe replaced, power no better. Over 2 weeks bottom end power rubish, but ok once turbo kicked in. Power got worse and eventually engine management light came on - fault diagnosed as P1901 - partical filter pressure sensor. Had this replaced - no improvement. Garage also checked EGR and whole of exhaust to to avail!!

HELP!