Thank god someone replied!
Basically I looked at this multiple times - adjusting the DEC clamp as tightly as I thought sensible and moving the end of the OTA back and forth slowly maybe 3 - 4 mm - trying to check whether the large brass DEC gear attached to the shaft was actually moving whilst the worm wasn't! The movement is very small - but I think it's there - so I perceive I'm in for a very careful dismantle - adjust and re-construct exercise!