Thanks for your advice.
I have been using a spreadsheet showing my monthly expenses to keep an eye on my spending etc. I have been using the same base sheet for about 5 years now. for each new month I have created a basic blank standard sheet by creating a copy before the existing sheet, deleting unneeded transactions from it and entering new data. (The existing monthly sheet is then copied into a sheet containing all the previous months for a particular year.
I think that somewhere in the past, possibly in 2016, one of these copied sheets has had the "fault"in it for some reason. This has then been forwarded on to future sheets being used.
I have now created a new sheet for next month (October) which looks like it will be functioning correctly. I have tried a couple of sample entries and the drag and drop seems to be working correctly.
Why the issue began is beyond me, but I am pleased to say the annoyance has been overcome (I hope).