Ah, right.

Behind the plate lies one of these -
http://www.ebay.co.uk/itm/NEMA23-75m...item4881441268

Its a standard Z-axis unit, screw drive, 2mm pitch, 75mm travel - the motor sits on top and drives the screw direct, the middle block is fixed to the gantry carriage and the upper/lower blocks move with the motor/screw.

Works well.