The iPad doesn't actually store all your email. All it is doing is showing you what email is on the server. If you leave all your POP email on the server, then it will show you what mail is there. However, after the initial access it will only alert you to new emails each time you check for mail.
POP email accounts are not really designed for access from multiple devices as usually the first device to access the email server downloads and then removes all new emails from the server, preventing access from another device later.
IMAP accounts (such as GMail) are much better for access from multiple devices (such as your iPad when away from home, and your computer whilst at home).
You may want to consider switching to an IMAP capable email service, now that you need to access your email from multiple devices.
So, the solution is to remove all my "old" e mails from POP server, but don't know how, or get BT to change my settings to IMAP. Not sure if they are able to do that but will send a request.
Simpler solution would seem to be create a dedicated Hotmail or GMail account whilst we are away. Don't really want to change ISP as it seems sledgehammer to crack a nut, so to speak.
If you access your BT email account on your PC or Mac through a desktop email program such as Microsoft Outlook or Apple Mail, then there will be an option in the account settings to remove/download emails from the server. If you download the old emails from the server to your computer before deleting them from the server, they will still be accessible on your computer.
You certainly don't need to change your ISP. Setting up an independent email account that is not tied to your Internet provider is certainly a good idea. If you ever do change your ISP in the future, having an independent email provider means you don't have to inform all your contacts of a new address, and change all your logins that use your old ISP email address.
You don't need to change ISP to use another email service. It's actually better to have an independent email, so that then gives you the ability to change ISP if you wish to. I use gmail. Just go to https://accounts.google.com and sign up for a google account. This will give you a large, IMAP capable email account that you can access easily from your iPad.
My E Mail settings show I have selected to delete messages from the server once I have deleted them from the computer.
If I follow your suggestion I should really tick the box to select delete from server after X days.
Am I correct?