200tdi boost leak/lack of power

Hey guys got a problem with my rangie. It seems massively down on power (bout 50%) The problem has coincided with turbo whistle and a very definate boost leak (its hissing really badly) im assuming its a boost problem and not fuel although it isnt smoking under full load like before, im assuming thats down to the fact its got a boost leak and not opening the extra fuel diaphram.
I've checked the obvious like intercooler pipes, intercooler, the boost diaphram in the pump and its pipework. I've also checked the inlet manifold gasket which was fine although its definatly not as bad since i put the manifold back on (couldnt get a gasket so had to temporally use instant gasket in conjuntion with the old one) I dont suspect the head gasket because it runs perfectly otherwise, its just not got its usual pull when the turbo kicks in and it wont rev past 2800 without really pushing it past 3rd gear.
Any ideas would be greatly apriciated as im completely out of them

Double post i know but it was getting lost in p38 questions as per norm

yeh thats why i use the general section..the rr section should be named the p38 section or just create a designated p38 section