The SD card reader is working again for whatever reason.
I have obtained my late brother's KN7 and will sell mine, as his has had very little use.
I also have about five other cards with a lot of data on them and will back them up for safety in future.
Following all of your advice I should be able to save the corrupted cards by copying my existing back-up card thru my computer, if I have understood you correctly and may not need to accept Mike's offer of assistance (Thanks Mike).
(Incidentally, my PC crashed on Monday without any back-up. I have now learned a nasty lesson, and it will not happen again as I will save all of my data to another hard drive. I did not lose everything , but it is warning enough to change my ways.
Also I have been advised to not remove the SD card while KB is turned on, which ensures that the reader will work effectively all the time using that process.
A big thank you to you all for the help and advice during this "saga"