Can you put traditional render on a house if it has already had an undercoat of K Rend on it ?

In theory you could - although I'm not entirely sure why you would want to. First you'd need to ascertain precisely which K Rend product has been used. For example, Silicone E-Grade would offer potential bond issues with any subsequent coats which were obviously now not monolithic. A polymer key coat would need to be applied (e.g HPX). If the K-Rend is, for example, HP12 or HP14 then absolutely no problem at all. All you need to do is observe the correct mix designation (extremely important!) and any cement based render would be adequate. Of course, K-Rend would only ever recommend that their products were used, and this is understandable because their BBA certificates obviously only encompass their own materials and rendering systems and not the cross-compatibility of such. The same as any manufacturer would insist. The precise reason for the question would be helpful. Then an exact and optimum specification could be offered. With so many technically advanced render systems on the market (polymer-mods, acrylics, flexible, anti-crack, breathable, nano-metric self-cleaning, etc) I can categorically think of no circumstance where I'd personally choose to apply a traditional 6:1:1 sand/cement render instead.