Peugeot 107 diesel power loss

Sounds frustrating! I had a similar experience with my old Vauxhall Astra, although it was a petrol engine. Reduced power, rough running... the whole shebang. Was your 'check engine' light on? And did you notice any difference in performance when the engine was cold versus when it warmed up? It could be a vacuum leak somewhere in the air intake system. Did you by any chance check the intake manifold pressure sensor?

Yeah, the engine light's been on for a while now, I should have mentioned that! As for cold vs warm, I haven't really paid attention. Where exactly is the intake manifold pressure sensor located? And would a dodgy sensor cause all these symptoms?

The intake manifold pressure sensor is typically located on the intake manifold itself, sometimes near the throttle body. A faulty sensor could definitely cause those symptoms, as it provides critical data to the engine control unit (ECU) about the amount of air entering the engine. However, given the hissing noise and jerking, a defective intake manifold flap control is very likely. Given the symptoms and mileage, I'd recommend getting it checked out by a reputable workshop.

Thanks a bunch! Sounds like it's beyond my limited skills. I'll book it in at the garage and mention the intake manifold flap control. Will keep you posted.