Have visited a Diesel Engine Repair Shop
I visited a diesel engine expert on the advice of a friend, just for a quick overview. He informed me that he believes that the cat is blocked and that I more than likely need a new one. I have had a quote for £150 for a replacement and fitting. I tried to get the old one off to see if I could clean it out, however the bolts are pretty stuck and their angle means that its difficult to get the leverage so I was worried I might round them off.
Though I can't see a 100% way to check whether this is the problem, I will double check with the garage that is fitting the cat to see whether they also think this is the problem. Again if anyone thinks this sounds about right I would appreciate the advice.
If I manage to find out I will post for others.