Mkiv Golf Turbo Engine Check light.

Hi all,
Im new to all this and im sure this has been covered a hundred times but I dont have vagcom and need pointed in the right direction!
Recently bought an Oct 2000 Golf Gti 20v turbo
Engine management light came on sat.
Went to autospark sat afternoon. Snap on machine brought up fault code -
17536 - Long term fuel trim, entire speed/load range, bank1- system too lean.
Possible cause- Fuel pressure/pump, injector(s), intake/exhaust leak, air system, hose leak.
Car seemed to be running ok and couldnt find any evidence of a hose leak, so fella reset the ecu for me and removed 25 quid from my wallet.
Didnt run very well after that so put bottle of fuel guard II in along with £20 of super incase it was anything to do with the £20 of **** petrol I got from Asda on Friday night. (it was really low on petrol when i got it, thought maybe dirt could have got in injectors?)
Seemed to be running better but after a drive of about 20 miles the light was back on.
Been on line searching for answers in various VW forums. Code 17536 seems to come up a lot, along with replacing the MAF sensor. So I took the neg. off the battery to clear the brain and cleaned the MAF with carb cleaner.
Went for a drive and its the same again- sort of hesitant and a lack of power espically in second.
I can hear a hissing from what sounds like the turbo boosting but theres no lift in it at all.
Light's off at the min, but I expect it'll be back on if it gets driven far enough.
Any help greatly appreciated!
Ricky.