Hi, I have a 2003 2.5 tdi l200, which I bought very recently...the chap I bought it off told me that there was a little loss of compression...I took a gamble and he explained fully that he had the car checked out and they found that cylinder 3 was losing compression(on last test they said approx 45-50% loss) the car has been well maintained and regularly serviced. Can anyone please advise the best steps to help rectify this issue and how to go about it. Much Appreciated. Thanks.

Do you have any smoke out the back of the truck?
Could be a number of things including stuck valve or worn rings.

A couple of questions...
Does it eventually start. ?
Does it run..?
If it does run, how does it run , smoothly?, lumpy and all over the place ?
These answers will help steer us along the road a bit...