Looks like you have hit the nail on the head with your comment on stress fracturing Gareth,
My mechanic says that at some time in the cars history it looks like a stud has broken and been drilled out and replaced by a pin that must be slightly misaligned as the first pipe to break was an absolute pig to replace,so he is thinking that with the car getting hot then cold it has caused the pipe to fracture,so today he is stripping it all down for access to hopefully remedy this issue and fit another new genuine egr pipe,
Fingers crossed for a successful result.