Focus Tdci Intermittent Miss Fire?

Hi, thanx for any advice in advance!

Ok start at the beginning.

Bought this 57 plate 1.6 tdci focus with 130'000 miles, no service history but running like a dream! As no history I thought I'd give it the basics, changed oil and filter, checked timing belt and could see it'd been done fairly recently, also notice the air intake pipe between turbo and the MAF sensor housing was split so changed that while I was there, But I don't know if it's just coincidence but after changing oil and filter it started to had a intermittent miss fire which got gradually worse within about 4 days to the point it died on me, no lights on dash but did come up with something like ' engine failure ' on multi functional display.

Had a diagnostic machine put on it and loads of faults but as the guy didn't know how old they were he cleared them all to start from scratch. Only 1 code came up witch was 'fuel pressure low/high '

At this point the car wouldn't run at all, just as if it was only running on 2 cylinders. Thought I'd change the diesel filter as I hadn't done that and it got rapidly better over a day to the point where it ran like a dream again!

That was about 5-6 weeks ago, until last Thursday ( it had just gone down to quarter of a tank if this is relevant or again a coincidence)

Now it has a very intermittent miss fire which could happen maybe twice, maybe 10 times in a 20 miles journey.

I'm wondering if any of you guys have had this problem before or if you could help me get my head around what's going on?

Thank you, sorry if I went on a bit but wanted to get as much info as I could in there