I eventually found what was responsible for affecting the recycle bin.
When I use the app Windows Washer which amonst other things cleans out the recycle bin, it somehow corrupts the bin. So when I next delete something, the bin appears to have something in it, but when it is opened, nothing shows up.
The solution is to do what TonyBose suggested in this thread i.e. after I have fininshed runniing windows washer and closed it down, then I create a file and bin it. Then empty bin. You may have to repeat the process, but you will know that the bin is fine once you look inside and see the file that you last put there.