If you go into admin mode and choose set access rights you can navigate to the global library and all your block folders. Point on a folder name you will see the id in the url.

Thanks for your response Peter. But i don't want to bind to an id which might change as we go through environments. Can I pick it up programmatically by the folder name? A folder search type function - i'm struggling to find the part of the API that would allow me to do that.

Many Thanks

You can read the folders like any other content. If you know that your folder is below the root folder and now the name you could do something like this.

IEnumerable<IContent> blockFolders = contentrepository.GetChildren<IContent>(ContentReference.GlobalBlockFolder);
ContentReference blockStartNode = null;

foreach (var f in blockFolders)
{
if (f.Name.ToLower().Contains(blockfoldername))
blockStartNode = f.ContentLink;
}

A bettor solution would maybe be to build a editordescripopr  and selectfactory class so you can make a nice dropdown on the starpage maybe where the system admin or editor can choose which block folder to use.

http://world.episerver.com/Blogs/Linus-Ekstrom/Dates/2012/9/EPiServer-7-Configuring-editors-for-your-properties/

Brilliant. Thank you Peter - that's exactly it. I didn't realise that folders are just content. Thank you