Nigel -
Picking up this thread and not sure I understand the problem.
If you have a R there is no surprise that the 45mm is a tight fit... Pretty much all SPS and R have a full system hence 50mm headers. You would need a 50mm link pipe (and not a 45mm to 50mm).
Also careful if you are trying to mismatch systems. The Corsa system is a complete different fitting to the standard system with the pipes fitting over the link pipe (and not into it).... typical Ducati that a Race bike has nothing in common with a road bike....
p
[Edited on 9-1-2004 by pguenet]