Thanks for your response, twerppoet. Poking around where you pointed me (I had been there in my hunt already), while it did not solve the issue it provided a little insight. It seems that on my iPad I have the usual inbox, sent, trash, etc. If I create additional folders there, they appear on my ISP mail account at the server. In the same way that my iPad will not let me delete those main folders that it provides, nor will my ISP server for its own similar folders, yet they are shown on my iPad under the iPads own folders. Hopefully that makes sense.
My iPad never used to show the server folders in this way. It recently began accumulating huge numbers of unseen mail, for example the inbox reportedly contained almost 30,000 messages, none to be seen anywhere. At first I ignored the number shown but my storage capacity was rapidly reducing to zero due to the mail. I researched and arrived at the solution to delete the mail account entirely then reload it. That action solved the unseen mail storage problem. My available capacity was restored but, since then, I have had this issue of additional sent, draft, trash, etc., folders.
I now have some understanding of what I am seeing but, sadly, no better understanding of how to resolve it. Perhaps this is a new bug because this seems to have reared its head around the time I updated the OS on the iPad to the latest version.