Hi Gill,
IIRC, there are a few very thin washers supplied with the carriage (about 10/12mm diameter) and these are used to shim the carriage plate (aluminium extrusion) up flush, or very slightly above the table surface if needed.
It's a bit fiddly as they are awkward to get in position if all four bolting points for the plate to the steel supports need shimming. (The plate sits to the supports with Bristol levers as bolts on mine if I remember)
It's worth it though, mine is still perfect from day one.
Andy