So: I have eventually managed to get to the sensor and give it a clean. It did look more clogged up than any of the others that I have seen on the photos.
On my Multijet 1.9 8v, just as BrianMcL suggested (thanks BrianMcL) I have removed 3 long bolts attaching EGR to engine, and that was it, it's created enough space for me to fit 40 torx bit and unscrew the sensor. Job done!

No probs, glad I could help.

Doesn't half make a difference to the car though, doesn't it?

I cleaned my boost sensor two days ago, same as you guys, mine was also full of dirt mixed with engine oil. Now the engine has stabilized, especially on low revs. Thanks very much for your help!

I am not sure if I can tell much of a difference now, but that might be due to other problem I have.
When engine was hot (slow moving traffic) for a moment engine started to sort of cough, lose power and revs and then carried on like nothing happened. Beginning of March alarm started to go crazy (I have mentioned that in other thread) and was sounding like crazy few times per night, eventually I have to close the door manually now. But before I started that practice, one day when I have parked the car just before I have turned the ignition off, amber light on dashboard and message check engine popped up. I thought after restart it won't come up, but it did and does every time now. Since that though now there is no coughing of the engine, all runs smoothly and acceleration responds instantly. I have cleaned sensor hoping that will get rid of Check Engine message, it didn't.