So my mate and i both reckoned the extruder wasn't 100% straight.

My first extruder carriage attempt had minor warp so got thrown.
Just re-printed and installed and the extruder now looks straight :-)

Also re-did the carriages with the bolts and washers the right way round. Much tighter and smoother rolling with no play.
Also sealed the base off with some cardboard and sellotape (to protect the wires) and just about to print some fairly basic feet.
Basically a round edged cube with cylinder sticking out the top that should push into the centre hole in the strut.

We're getting there - one of these days I'll even try and print something. Still got to clear space on my desk.