I think you can use any Clarendon house with render finish as your reference sample. You can also use any other house with render finish as your reference sample as they all (should) have the same level of quality. Alternatively an independant inspector will provided a documented evaluation that can be taken to the Dept of Fair Trading (or equivalent).
In the end, I would simply go to a Clarendon display home (or completed home) with a rendered exterior, point to it and say, that's what I want.