In an idle moment I was browsing through my external HDDs to ensure they still had sufficient free space and eventually got to the one that contains my Lightroom catalogues.
When I opened the folder containing the catalogue that I currently use, I found a sub-folder named "Backups".
Looking into that backup folder, I found a whole series of sub-sub-folders; one for every back-up Lightroom had made of the catalogue since I started using it. Each sub-sub-folder contained a backup of the catalogue file, each of them over 600,000Kb.
That seems an awful lot of HDD space being used unnecessarily.
Is there any setting in Lightroom that would tell it to retain only a certain number of backups (say two for safety) and automatically delete older ones when it saves a new one?