I'm in Berkshire, and I've found a local rep for Carlsson Autotechnik, who can do 3.6 and other engine mods for the 103 inline 6
here is the link to Carlsson, http://www.carlsson.de/indexdswf.html
(i got this link from tirerack)
and here are the details of their local UK rep.

I'm still investigating if there are other local to me MB tuners.

Russ