I downloaded some free gimp brushes, I think they were converted from other formats, some how the file name (which is descriptive e.g. Mud_01_H.gbr) and the description (a number eg #46) which appears in the brush selector dialog are different. The thumbnails are small and most fine detail is difficult to make out making it difficult to select a brush. Is there a way of changing the description without having to open each brush as an image and save as?

AFAIK, you would have to open the brush in Gimp, export as a gbr and rename it like that. Sorry.

The filename of a GIMP brush is basically meaningless other than the extension. What you call the "description" is actually the brush's name.

Unfortunately GIMP does not allow you to rename a brush, so (to my knowledge) you are stuck with opening the brush as an image, Edit->Copy, and then "Edit->Paste as->New brush".