D5 Swirl Flap Assembly replacement guide
 

Some photos and useful info I noted down while replacing the swirl flap assembly on my 2008 D5244T4 engine.

Inlet manifold is one large plastic moulding. It looks like the front/top section can be removed via 4 bolts, but it's not removable and those 4 bolts are part of the overall attachment to the cylinder head.

All 5 injectors and high-pressure fuel pipes must be removed before the inlect manifold can be removed. Injector 4 was really stuck despite not looking too bad:
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I left the well soaking in Wynns inlet/carb cleaner for 2 days with lots of heaving 'tapping' left and right with mallet and socket extension bar. Eventually got a few degrees rotation. Ran engine with bolts loose but never enough to 'pop' injector out.
I accidentally broke the solenoid nut trying to rotate the injector, so I ended up having to buy a new injector (Darwen Diesel were brilliant, highly recommened). This at least meant I could then dismantle the injector further and fit a proper extractor:
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The key tool here was a hollow 10mm allen key to remove the inner nut, and make the internal 17mm threads accessible to the extractor tool:
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Before the inlet manifold could be lifted up, it was necessary to create space by moving the hose bracket behind fuel pump so that the corner inlet manifold bolt could be accessed. The oil filler spout also needed to be removed. It was also necessary to loosen the 17mm banjo bolt on fuel pump to move hose out the way.

The inlet manifold was quite hard to lift up simply due to the rubber seal being stuck in place (it's a great design though). It's also necessary to first pull the large rubber connector hose off the main air intake at the front.

Once off, remove the rubber seal and start cleaning !
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The control arm on mine had become very worn and wobbling/sliding around on the main shaft. I actually think the cause of oil leak is between the outside of the control arm and the plastic shaft mounting. Compared to the new assembly, this part was very worn. It's not a great design, especially compared to how good the seal is underneath this mount (see photos), but in theory there shouldn't be any oil getting into this region of the engine. THe amount of play between the control arm and main shaft meant when moving back and forth, the swirl flaps were only opening about 2/3rds of the way. So, along with the fact all the flaps were caked in sooty oil, I'm quite glad I decided to replace this part.

Next job is to lever the old swirl assembly up out of the cylinder head.

Caution:
The white plastic end bushing on the old swirl assembly was so worn it fell off as I was lifting it all out - I just managed to catch it before it fell into the air inlet port
Don't forget the cylinder head is aluminium and will scratch easily if you're cleaning off grime with a flat screwdriver - best to use something plastic if possible

Clean as much of the black grime off the cylinder head as possible. Install the new swirl assembly and line up the plastic link arm to the motor ball joint and rotate the swirl back and forth and observe the stop points. This might be useful to have seen if you have any problems with the link arm and motor.
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At this point, you could choose to drill through the swirl arm ball in case you want to fit a bolt in future (I chose not to do this in case I had to return the swirl assembly).

Connect the link arm.
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Things you should/could do at the same time:
Inspect the cam surfaces for wear. Inspect the valve and springs for any signs of damage or discolouring
Inspect the wiring loom running next to injectors as the outer housing wears out quickly against inlet manifold:
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Clean the EGR valve assembly and pipework. This can be a horribly grimy job. I removed the air filter box, main air intake pipe to the RHS of the EGR valve and fitted a clear plastic bag over the end of the housing. I then spent an hour spraying carb cleaner through the from RHS of the valve housing, lots of scraping and pushing through balls of kitchen towel with long screwdriver. I got it as clean as I was willing to, and until I eliminate the source of oil (turbo seals ?) there's no point making it spotless.
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Clean the inlet manifold: I used loads of Wynns Inlet/Carb cleaner and scraped out dirt. Then jet-washed inside large plastic bag.
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After drying, fit new rubber seal to inside of inlet manifold
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It's worth preparing the injector seats at this point. I made a 'cleaning stick' by cutting a foot long 18mm dia dowel and turning the end down to about 17mm dia.
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I made a thin pipe attachment to my vacuum cleaning and gave the injector seats a good clean until near spotless:
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If cleaning pipe to EGR valve housing, need to decide whether to refit this pipe before putting the inlet manifold in place, or afterwards. It's harder to refit afterwards but the advantage is you're able to gently fit inlet manifold in place without needing to push fit the large pipes together.

Refit the inlect manifold and tighten bolts to 10Nm, starting near the middle and working outwards.

High pressure fuel lines - need 15mm crow's foot and torque wrench accurate for 28Nm.
Injectors, need 8mm socket and torque wrench accurate for 13Nm.

Once everything was back in place, I first cranked engine for 10 seconds and then inspected fuel pipes for leaks (all good). I then cranked engine for another 10 seconds and it started very nicely. Tiny bit of smoke out exhaust which is understandable given all the carb cleaner used everywhere. Took car for a spin, difficult to comment on improvement yet - it certainly felt very nice, and I'd like to think it's breathing easier and revving better, but that could be placebo. I'm hoping fuel economy will have improved, particularly in traffic where I'm now hoping the swirl flaps improve efficiency. I've not actually checked to see if the swirl arm motor is moving yet. I might just try with VIDA to make sure the motor is actually working.

As an additional extra, a couple of useful videos from other people who have done this same job:

Demonstrating a worn swirl assembly and oil leak: https://www.youtube.com/watch?v=DNHt9mcO2rI
My Korean pals showing how it's done: https://www.youtube.com/watch?v=Iug5-8WHb2s