turbo issue?

hi all, i have a 2004 1.9 tdi 100 which was my dads taxi for an endless amount of miles and since him passing it down to me when he upgraded its been a great running car.

But a couple of weeks back i drove home from work and then noticed a lack of boost from the turbo. it felt as if i left my turbo at work and it has been completely slugish since that night.

It has had a new MAP sensor and a new boost pressure regulator fitted by myself last week as i had a local garage i used to work for scan it to check what the management light was caused by and that said n75 - boost pressure regulator. 

Since then i had the light cleared after i fitted the n75 and yet it hasnt resolved the issue. Any more guesses what is causing this?

I have noticed a split in my flexi on the exhaust? maybe the cat is blocked and causing a build up of pressure somewhere but i dont know much on this subject so hopefully one of you guys could help?