After 2 local garage visits and one Citroen dealer visit I managed to determine the root cause of the issue (none of them could help me to correctly diagnose the issue).
As I mentioned before the 2 P codes I got were the outcome of all issues with my car.
Both related with the fuel pressure signal. As soon as I realised where the fuel pressure sensor is located I knew what the issue could be. And it was a chip tuning I plugged in 3 years ago between the ECU and the fuel pressure sensor. I removed the chip and all the issues disappeared
. Of course my car was still in limp mode (but no MIL light on the dash).Must have had the issue with it (chip) for a while as due to incorrect fuel pressure signal from it (I believe) and as I presume too rich fuel injection it caused my DPF filter clogged, again! (I changed it 6 months ago along with EGR clearance).
Now bit of the garages I've been so far. First one, nearest to my house, they said it's the DPF clogged, said they'd tried forced regen but didn't help. They couldn't see any other fault codes rather than clogged DPF. Thanked them, paid £65 and left.
The second one say about themselves "Diesel Specialists" in Loughborough. I thought: "These guys will definitely help my car". Left the car with them, got a phone call few hours later: DPF filter clogged, we can try forced regen but that's gonna be £150 plus VAT! They said they didn't see any other faults rather than the DPF fault. I tried to explain them that the clogged DPF is not the root cause of the issue and is clogged due to some issues with the engine's combustion process. I gave them 2 P codes I got from my scan (smart phone app) and asked for some investigation around these faults. They came back to me saying the codes I gave them wouldn't cause the troubles with my car. They also said the DPF filter is sat incorrectly and there's a leak of the exhaust gases from it and that's why it clogged and that's where I hear a whistling sound (I mentioned them about some strange, whistling sound coming from under the bonnet). I thought that this could be the case as I could f**k something up when I was changing my DPF 6 months ago. I thanked them, paid £45 and left. Came back home, cleaned my old, genuine (I'm glad I kept it!) DPF with pressure washer , dried it and put it back to car. Tighten all the bolts really solid, took it to a test drive and...still the same; smoke, noisy engine, revs jumping up and down, stalling, engine shutting off and...whistling sound from under the bonnet. I was absolutely devastated
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so decided to call Citroen dealer and booked my car. Took it to them, said about all the symptoms (I didn't mention about the fault codes as I was sure they'll see them when they plug it in to Diagbox). Got a call back few hours later and...EGR needs changing! If I wanted that would be only £560!(including VAT). They also said the whistling sound is coming from it as whoever put it (it was myself ) did it wrong and there's a leak of exhaust gases between the EGR and cooler. They said they aware is expensive but this part is hard to access as is at the back of the engine and is gonna take some time to take it off and put it back 
and the EGR itself is quite expensive as well. When I said to him that this is one hour job for me as I did it before and the EGR itself costs £100 for brand new, genuine one on ebay, the bloke's face expression...priceless. I was really pissed off! Couldn't believe they tried to rip me off as I was some blonde girl! Asked them what fault codes they got when they plugged it in they said: none/nil (0)! Thanked them, paid £75 and returned back home. Removed the EGR, had a look and I knew there was nothing wrong with it (I cleaned it 6 months ago), it was running really smoothly. But indeed there was a leak of the exhaust gases as the gasket was covered with soot nearly all around. So I put it back, this time I used high temperature resistant sealant, removed the chip tuning took it to a test drive and spot on, happy days! Of course the car was still in limp mode so I bought Diagbox, scanned it, cleared all the faults and is running as a baby again . End of story.So now I have diagbox v.7.57 fully working if anybody needs some help you're welcome to pop down
. I'm still learning about it so I'm not an expert with it.Again, apologise for long text but hope can help someone in the future.