That may well be the cause of your problem.
This is a bit technical so I will try and keep it simple.
If your iPad is set up as an IMAP email account that means that it synchronises and replicates your files/ folders and emails that you have when you access your BTYahoo email account via your web browser.
If you delete anything on the iPad it will be deleted on the BTYahoo servers and as such you will not be able to see them when you check your BTYahoo email account via your browser.
The same applies if you delete anything from your BTYahoo email account on the web browser. This will be replicated on your iPad and as such you will not see anything that has been deleted.
Hope that makes sense.
I admit I am struggling a bit but I get the sense of it. I have not deleted these mails. They have disappeared by themselves. Apparently it happened to another user some months ago and the link I am searching for worked for him too. I think it was to Yahoo not BT who washed their hands of my problem completely- no surprise there as they are the worst company in the universe!
I want to sort this problem before changing to Sky as my e mail provider.
Is the sent items folder just gone, or is it empty?
Also, I would recommend switching to gmail or outlook for your new provider of email. Not only does this make it less annoying to switch providers without having to pay, these services usually are much better. Also Sky uses Yahoo for its email so you may have similar problems.
The sent items file is just empty. Thanks for the tip about Yahoo. I will keep trying to retrieve the mails.