thanks for that.....now, one more :)

is there any way of getting just one (random) DataObject through this method?

ie can you pass anything else to $theSection->Banners("BannerType = '$bannerType'");

Yes, you can.
If I remember correctly it's:
WHERE, SORT, JOIN, LIMIT

To get one random Banner you could call:

$theSection->Banners("BannerType = '$bannerType'", 'RAND()', '', '1');

Just wanted to say that I continue to be blown away by how awesome Silverstripe is.. I'm running in circles trying to get some DataObjects all working together and then with a quick search I find out that I literally need to code up:

		$d = $cat->Businesses(); 

Just.. wow :)

Thanks!