OK folks, I've had a reply from BBC Info. The Treasure hunt has officially finished but tapes can still be returned to them. The gist of the reply is below:
...However, although the campaign has closed this does not mean the search has finished. BBC Archives would still be interested in hearing from members of the public who have BBC recordings of interest to offer. You can contact them at the following address:
BBC Archive Treasure Hunt PO Box 3087 London W12 6GL
If you intend to return anything, please write first and tell them what you've got before you send any tapes to them. If you get any positive response from them please let us know on this forum.