After some judicious wobbling I have found the problem !
The bearings that run up and down the smooth guide rods are simply clipped into the plastric assembly.
There is nothing holding them steady and they rock in the sockets. Presumably they should have been secured by cable ties.

So It looks like I'll need to remove the extruder assembly again and wither drill some holes for cable ties, or glue the buggers in.

Autowhiz, I noted you'd 3d printed your parts - got the stls anywhere ?