The answer turned out to be surprisingly simple. It came from Nitin on this Forum: Press Windows key + R, type SystemPropertiesProtection, and press Enter.Navigate to the System Protection tab.Select the drive from where you want to delete system restore points.Press the Configure button.In the next window, press the Delete button. I had in fact checked here hoping to restore to an earlier state, but as system protection was 'off' I didn't think it worth pursuing. How wrong was that! Once I deleted the restore points, all the missing space, 130Gb became available. The next morning Windows 11 kicked in and installed itself without a hitch.  People with low available storage may note that the previous installation of Windows is retained and takes up 16Gb of space. If you can't afford to lose it, that 16Gb can be recovered via file cleanup.  Of course if you do this, you cannot restore the earlier version.
I have an Inspiron AIO DT 5400 with a 220Gb SSD drive which I use mainly for web meetings. It has no data and only a few apps, Microsoft 365, Adobe Acrobat DC, Photoshop Indesign and Zoom. I recently started getting warnings from Windows to free up more disk space. When I checked there were only 375 Mb reported as free in Explorer. I have since managed to get the space up to 8 Gb by deleting a few things and turning off hibernation, etc. Total space usage is reported by Spacesniffer utility as 54 Gb yet the same utility only reports the same 8Gb of free space as Explorer does,. Tech support at Dell will not touch it as it is classed as a data issue. I am left with two possible assumptions a) some kind of malware infection b) Drive fault. I also suspect that abortive attempts to install Windows 11, instigated by the PC itself, have left some bloated files around. When I first ran Spacesniffer it reported around 100Gb of system files. It is no longer reporting these, but is not reporting any recovered space in spite of that. I would be most grateful for any suggestions on how to proceed to recover at least a working amount of space. Best Regards Rob Butler      
SSD Drive reported full by system when only 25% full