Okay, so just in case anybody has been looking here anticipating an answer and been disappointed. I have managed to figure this one out - at last!
Here is an example:
y = peaks;
h = pcolor(y);
h.ZData = h.ZData+5;
This will give a plot with a pcolor representation of peaks at z = 5 and another at z = 0 (the default for pcolor plots).
I hope this might help somebody one day!