Just thought I'd post my experience as it might help someone.

I think I'd started to notice the car running badly first of all.

Lumpy idle 

Also lumpy around 1800 revs, jerky, bouncy, kangaroo car.

Then hissing noise coming from engine under load. Worried it might be turbo.

Black smoke kicking out the exhaust

Warning triangle on dash with warning chime "ENGINE MALFUNCTION".

So, I got around to opening the bonnet (I need to find the correct long screwdriver for this task). I notice a lot of black, around the area a large pipe/hose connects to the rear of the top of the engine. I believe I may be correct in saying this is the inlet manifold.

I put my hand around the big pipe, but slightly knocked a small, u-shaped pipe above the big pipe and noticed it was cracked. I decided this needed replacing. I turns out it's the MAP sensor pipe. This is unavailable on its own, without buying the sensor.

I purchased some 4mm internal diameter vacuum pipe and cut a nice length to fit. 

This did not have any effect on fixing any of the issues. It obviously needed replacing though.

I then went back to the big hose, ordered one from Amazon and replaced it.

On the underside of the hose, there was a big tear.

Shazam.

Fixed.

On my engine, it's was a 90 degree pipe, with 45mm internal diameter one side and 50mm the other.

Afterwards I looked into getting silicone hose instead of rubber. It seems this might be a decent upgrade and doesn't seem to cost more. Longer lasting and looks better. Available in different colours, but I've been advised to stay black, as your insurance company could point it out as modified, if blue or red etc.

I did notice that further down this pipe assembly, the next rubber pipe, which I believe goes to the intercooler is looking quite ropey, so maybe I'll try and get that section in silicone. 

Yup - That little 90 degree hose was quite infamous back when the 1.8 TDCi was a much more common engine. Speaking from personal experience, replacing the original hose with an ebay silicone 1 meant I never had the issue reoccur.