Try turning off your BT hub for half an hour and when you turn it back on you will probably be issued a new IP address and hopefully be connected to a different BT email server which may allow the email to be deleted.
Thank you for that suggestion, I'll try it out.
This is just one email, but since BT upgraded their mail programme, I have other problems, such as unable to complete the request, timed out (and this after just 12 seconds !
I often have to close the programme down and start again.