Thanks for your detailed reply Eeeps, I have to say I didn't think it was all going to be so complicated! I have eventually managed to successfully set up IMAP email accounts on both my iPad and iPhone, despite the wrong port number being entered automatically for sending messages, but I figured it out in the end and it works! Re your last paragraph, I have noticed that the emails don't get "pushed" quite as quickly as before, but they get there in the end and I can live with that.
The only minor issue is that when I delete a message from the iPad, it now disappears from the server and therefore won't appear on the desktop, (which is a POP3 account) which I would like it to do. I assume there is a setting I need to tweak in my VM mail account settings but can't find it, any help gratefully received.
As to my mother-in law's problem, she is coming round later (it being mothers' day) and hopefully I will be able to set up an IMAP account on her iPad, now that I know how to do it, and that will solve that. Fingers crossed.
Why is nothing in life as simple as it should be?