Alrite pal,
which tddi do you have the 75 or 98bhp,. If its got the intercooler then id check all the wires to the maf and intercooler sensors and probably give them a good clean too.
p.s how quick does it rev to 4500 it should be instant while stood still, if it builds up slowly then deff turbo problem but a much more precise way to tell is to get up to about 40 in 3rd release your accelerator and put straight back down you should deff feel the turbo boost kick straight in because theres no wind up time in diesels
Do you have any other problems like difficulty changing gears especially between 1st and reverse, grinding nosies etc because it sounds like your engine producing the power but its not getting to your wheels. id think about checking gearbox oil, clutch bleeding (ur brake fluid feeds both brakes and clutch through seperate channels)