I am not sure that you can rely on the extension telling you very much. I only have the SONAR 64bit X series installed. Most of the DX stuff on my machine are DLLs, for example everything in the "Shared Dxi" and most everything in the "Shared Plugins."

Use the VST version. Many or most DXis don't work in 64-bit Sonar. 

OK, finally cracked it! Thanks everyone for pointing me in the right direction.

It was nothing to do with the version of REX shared library at all, i simply reinstalled RXP (and PSYN II which had somehow lost its presets) via the advanced install function and it worked.  It was me being a bit dumb because it does clearly say on the release notes for X1 that if you are upgrading from a previous version of Sonar you may need to re-install some plugins and it seems the DXI's are the ones most most affected. 

Reinstalling the plugins wasn't as easy as it should have been though as I have X1 Expanded and to do the advanced install had to go back to my X1 disc which has a different serial number to the expanded update, cue all sorts of hassles with serial numbers and having to modify the registry to get the right one displaying in Sonar!

Anyway, all is working now so many thanks for your time and advice.

Ben

That is great, glad you got it sorted out. Yeah, X1d Producer Expanded has to be the most convoluted install of SONAR yet.