I cleaned the MAF thoroughly yesterday and this made no difference. If I disconnect the MAF there is no noticeable difference either. I have looked at the lengths of vacuum pipe and the cotton braided covering is damaged in many places but can't see any defects in the rubber underneath but I have ordered some lengths to replace anyway.
I tried to bypass the vacuum reservoir by connecting both pipe ends together with a piece of 3mm plastic pipe but the car would not start when i tried that so not sure whats happening there - I'm guessing it should at least start unless the seal was very poor maybe?
I'll check the EGR valve when I get some time.
Thanks for the help with this.......

Well,now you know its not the MAF.
Very weird why the car didnt start when you bypassed the reservoir!Maybe the 3mm pastic pipe blocked the vaccum !
With regards to vacuum ,its best to do a leak test ,rather than change pipes by trial and error as there are a lot of them ,also under the inlet manifold ,going to EGR solenoid.You need to buy or borrow a vaccum gun .Its a very useful tool and imperative in fault finding problems with the diesel vast vaccum system!If you get a gun,I will guide you through the testing,no problem.
You can get one off Ebay for less than 20 quid and bleed the brakes too!
Hand Held Brake Line Pipe Fluid Pump Vacuum Bleeder Bleeds Car Bike Garage Tool | eBay

UPDATE......

I tried bypassing the vacuum reservoir again and car started fine this time - but ran poorly.

I swapped out the degraded section of vacuum pipe - car ran sweet for a few miles and I thought i'd cured it but then it reverted to lacking revs again.

I popped in to my local mechanic, he read the codes and said it's probably the MAF sensor. I told him I'd disconnected the sensor and no difference, and that I'd cleaned it but no difference - he still thought it was likely to be MAF sensor.

Took it into another mechanic, he said it's gone in to "limp" mode, but surely not that as sometimes it revs very well.

On friends recommendation I have taken it to a BMW indy who thought just on my description it would be a fuel flow issue. He has just called to say it was the middle of the 3 fuel pumps failing. Although I'm sure I could have swapped this over myself the indy said he was 100% sure it was that and as the covers where of it would be a very quick job for him to swap it there and then so he has apparently fixed it. Just on my way over there to pick it up - fingers crossed and hope the bill doesn't hurt me too much.

IMO cleaning a MAF rarely works. It doesn't hurt to do so but don't count on it to solve your problem. Also, IME, if you disconnect the MAF and you continue to have issues then the MAF is not the problem. Disconnecting the MAF will cause the computer to use default parameters but those default parameters are sufficient to properly operate the vehicle.

I've cleaned the MAF many times. It has always worked though I have found the necessity to clean it becomes more frequent. Now I clean it once to verify the MAF is the problem and order a new one to have on hand when the MAF acts up the next time.

Hi Treeman,
Any updates on your problem?

X5 running sweet - I forgot how nice it runs when everything as it should be.

Indy changed the middle pump and all is ok.

I have been told before by my BMW dealer mechanic that my age X5 (2003) can throw misleading error codes (i.e. Air pressure code displayed when it was the fuel pump failing). He explained that in 2003 they where only just starting to use computerized error coding and although sometimes helpful he said they where often misleading. This was when one of my injectors failed but the error codes thrown up then said nothing relating to injectors being at fault.

Anyway thanks for all the help. especially to the two of you who did mention Fuel Pump as a possible cause - It does show though how misleading the error codes can be as having mentioned the Air Pressure error code and the first 3 mechanics diagnosis that it must be Air pressure related I pretty much forgot to pursue the Fuel Pump possibility.

Diagnostic codes are meant to assist in locating a problem. They are set when a sensor detects an out of specification condition.

Without knowing the specific codes you had I can speculate a low pressure fuel condition could cause an air monitoring circuit to read an out of specification condition. As fuel pressure decreases the amount of air in the air / fuel mixture could increase outside of acceptable parameters. Often times this results from an air leak and thus a misdiagnosis occurs because the mechanic is investigating only one of the causes.

I had the same problem a few months back and it turned out to be a bad elbow tube which connects directly into the throttle body assembly; there was a hole in it.

Item #8... http://www.realoem.com/bmw/enUS/show...diagId=13_0903