Thanks for all of your advice. Here is what I did:

From a totally un-listenable reel-reel tape (50 years old) copied to a cassette tape - this latter was recently digitised. The hum actually came from the dubbing onto the cassette. The reel-reel tapes are long out-of-reach. But the hum was so noisy that the music could hardly be heard. However a combination of Reefer (using ReaFIR) and then Noise Removal on Audacity - and the result isn't bad. Some links:

http://homerecording.com/bbs/user-forums-brand/dont-fear-reaper/noise-reduction-reaper-269007/

Basically I had a WAV file from the cassette tape. This I read into Reefer. Found a gap in the recording where the hum only was present - actually the gap created by the cassette turnover event. Using Reafir I created a noise profile and applied that to the recording. I saved the result as a FLAC file.

I wanted to repeat this for the residual hum. But Reefer was producing a crackling sound when I re-opened and tried to play the FLAC file - so I abandoned that idea.

I then read the FLAC file into Audacity. I applied Normalisation, Pop Removal, and a speed decrease of 2% (Reefer didn't seem to allow for these).

Then I found the same gap with the hum and copied this into a new file with Audacity. Then I scanned that with the (Nyquist) Noise Removal effect. Then I applied that new profile to the whole file.

Finally I saved the result as WAV. FLAC and MP3 (320 kbps).

It is the latter which is uploaded here:

London Folk Song Cellar programme 10 (of 39).

LFSC 10 - TD 115636.37 - RMcL c4 (320)

https://www.firedrive.com/file/59103CF9FBE1E4A4

https://www.firedrive.com/file/02EB57D94EC51E00

CJB

P.S. I will also give SoundSoap a try later.