Thanks for that Adrian, much appreciated.
I've contacted the company in Germany and it looks like it's just one guy doing this on his own. Unfortunately he tells me that he hasn't been doing any for the last 12 months due to ill health. He didn't give any indication if he was planning to return to this or not. He did give me a link to a colleague though.
https://www.katalysator-matrix.de/kontakt
I took a quick look at the site and to me it just looks like a manufacturer of different sizes and types of catalytic converters rather than a refurb setup.
I really don't know enough about this subject to be able to say if that's any different from what we can get hold of here in the UK.
Unfortunately it does look like the rebuild option has now possibly gone unless we can find an alternative.
I'll keep looking
Cheers everyone.