surely that is a disincentive to get a supply only kit, as you will pay 21% on the kit and 13.5% on the installation? Whereas if you bought the erected kit you would only pay 13.5% on both. Doesn't seem to make too much sense.
On a slightly different note I would be cautious to get a supply only kit. For example, how do you deal with a situation where a problem arises which the builder claims was caused by errors in the manufacture and the manufacturer claims were caused by errors in the installation.
J2K