Among my many duties I've been an IT Coordinator since 1984, so I've heard the weeping and wailing and gnashing of teeth far too many times - got FIVE generations of backups for this!! 

So, I've done your instructions in the GT for the roof, (only 2 polygons) and sure enough on the Material room menu I have Preview and now Roof Slates, click on that and go through the New Node>etc procedure, browse to a slate-ish texture and sure enough it appears on the roof. If I export and import it, it comes back with the same texture on it.
But then I did the same thing with another of the backups without bothering with the Grouping tool at all and got exactly the same result. 

So why am I going through the Grouping procedure? Can I not just UVmap a simple object, cut and paste the texture till it fits the UV template and then apply it? Does UVMapping, by telling you where the polygons are, allow you to do the same as the GT? Is it only with complex structures like people that you need to use the GT? Someone please comfirm that I'm not missing something here.

Thank you all for your comments, it has certainly been an interesting day so far (I nearly died when I saw the Renderosity Home page this morning!!!)

Colin_S(mith), definitely not any sort of guru