Engine hesitates while accelerating
 

I've just come into possession of a 55reg S40 2 litre diesel with 97,000 miles on the clock.
It runs rough and when accelerating the car hesitates mainly between 1200 & 1800 rpm. Feels like a flat spot and can happen in any gear. Under hard acceleration the car pulls well. The issue is very noticeable when cold. Like running on kangaroo Juice!!
When viewing the clear fuel pipe that exits the fuel filter to the fuel pump, air bubbles can be seen occasionally and are more noticeable when revving the engine. The outlet connection is currently being held onto the fuel filter with a cable tie! I'm thinking this is the main culprit and is letting air into the fuel system and causing the poor running? Could it be anything else?
Where could I obtain a new pipe? Expensive?

Btw, car starts fine and there are no error codes

Sounds like the common boost hose split on the left hand hose running into the throttle body.
Usually the case that there's oil that's been misting around in the area.
There is a collar gasket which the plastic pipe sits over and this tends to fail or get mangled.
On my old V50 2.0D I use some self fusing rubber tape as a replacement and it worked a treat.

Also the boost solenoids are known to cause this issue, there two of them and are identified with vac hoses leading to them and a brown case, the harder to reach one is near the alternator so some bits need to come off to reach that one.