Engine light and carbon problem

Not quite sure where to start and do not want this to sound like a downer on the X3 because I actually like it - just plagued by the carbon build up problem (I think).
Bought the X3, 2.0 SE diesel in 2006 when 6 months old and had good service from the dealer until the warranty expired, which is when the yellow engine light started to come on and briefly after 3 years of trying to resolve still no success.
Last year spent a fortune replacing various parts that the dealer recommended should be changed, EGR, sensor valve?, Air mass filter....etc. finally took it to an independent who instantly said the inlet manifold needed cleaning which he duly did. All this worked for a while but for the last 6 months the light is back....miles per gallon has reduced again to around 25 daily and 45 on a run. Used to do 35 daily when bought.
My driving pattern probably doesn't help as its mainly 10 miles to work and 10 miles back although I do give it a long run once a month (200 miles) and try and give it a high rev cycle (~4000 revs) for about 10 mins every week.
I'm waiting for something I saw on the web from "power enhancer" BG244 diesel fuel cleaner - I'm not 100%confident this will sort out the problem. I will even start doing more regular oil services myself than the indicator.
Just looking for guidance on what I can do as I intend to keep the car as its practical and I do like it been though its a hard ride.

I an considering either replacing or cleaning the inlet manifold again - is this something that a practical DIYer can do? If so is there a good step by step guide or could someone help me out with a guide.

I used to do most of the engine work myself on my earlier cars many moons ago but have been softened up by the newer cars.

Are there any other products that I could used to clean out the air intake side rather than the file end?

Any help or guidance appreciated as I am loathe to go back to the dealer again and the independent although good still wasn't able to resolve this - or am I asking too much?

Cheers and thanks