I followed the http://wiki.contribs.org/RecycleBin howto and installed the automatic cleaning script as per the instructions. However, as written, the script doesn't remove any files older than 30 days. It only works on our servers if you set the -ctime parameter to 1 or 2, I checked this on SME 7.6 and 8.0. For some reason the last changed file attribute isn't appropriate. So I changed the '-ctime' to '-mtime' and the script now functions as expected. I don't know if this is a quirk of our servers but thought I would post in case it helps anyone else as it's a great feature.
Ok, I have taken courage from your suggestion and requested access to the wiki.