Thought I'd round this off.
After much to-ing and fro-ing, with me answering lots of detailed questions about my car, they agreed to collect (at their expense) the exhaust from me.
It took a while but I now have all of my money re-funded. So, whilst I could definitely have done without the aggravation, I have the outcome I wanted.
What I couldn't understand with the Eisenmann exhaust was the presence of the thin HE flanges. Interestingly this is the response given to me post collection of my system:
'In case any interest at all, they did previously discover the issue of the non fitment 6 – 8 mm gap was due to differences between the cylinder heads between USA and European specification cars, they do offer a system for your specification with different flanges, the error was in not remembering this time to ask you for finer details / VIN of the car before hand, apologies for this; it is the first time such a mistake has been made that we are aware of on any same units we ourselves have sold, sorry it had to happen to you.'
This from Eisenmann UK. Now I don't believe this is correct at all - I do not believe that there is any cylinder head difference between UK and US cars, but am happy to be proved wrong.
My understanding is that ALL 3.2s are thick flange HEs.
The Eisenmann website has now been modified to state this:
This should fit the following; 3.2ltr engine 152,160,170kW - WITHOUT cat converter, however some modifications may be required during installation on these 3.2ltr engines.
Please provide dimensions of the OEM heat exchanger flanges before ordering.'