Update on this. I changed over the Governor and manage to regain 3rd gear.
I decided, however, to go back and fit a self adjusting servo and the original valve block with relative kickdown cam assembly and cable. There were a few details that differed from the workshop manual, firstly, the rear pump valve was located to the front of the valve block and there was no shim set on the downshift valve (part no 12 on the exploded diagramme in the WM)
When driving on the new valve block, cable cam and servo it appears to hold first for much, much longer. I've heard the original 3500s were considered a bit more spritely off the mark, is kickdown cam profile responsible for this? the change from 2 to 3 appear effectively as normal.
I'm not overly convinced that the 1 to 2 set up is quite correct. It seems to hold 1st until about 4000rpm on a medium throttle but the power fades off aroud 2500rpm as if the box is slipping? I'm not sure whether this is actually the case though.
Thoughts and considerations welcome..