Thanks for that info Peter and an update.
I think I have the correct spacers (7.1mm) in there, but I have tried inverting the stepped rubber washers and putting then in the holder from the top. Tried this loose with good results and will try with an additional rubber washer on the outside. But much improve.
Paul, I see you have additional thoughts on your mind (exhausts) and I will be following that with interest as I have been running a set of the repros from DSS for a year.