We took off the first catalyst and found it to be blocked with soot.
I had already fitted a new EGR valve which it blocked with carbon on a road test.
Final remedy was to clean out the catalyst, clean the new EGR and refit, replace the split turbo hose and of course replace the thermostat so the engine got hot enough to do a natural regen, which it did on a motorway roadtest.
Thermostat failure on these is quite common apparently but of course combined with an airleak and a faulty EGR making excess soot and the low temperature preventing DPF regeneration from happening it was bound to end in tears.
Shaun.