One of my colleagues has PlusNet mail working with his HTC desire. He's pretty sure he used the manual setup option.
Following the guide here :- http://www.knowyourcell.com/htc/htc-desire/desire-guides/443083/how_to_set_up_pop3_and_imap_email_ac...
Enter your email address and then select 'manual setup'
You can use either POP3 or IMAP but I would suggest IMAP since the Desire doesnt seem to have any option when using POP3 to leave messages on the server.
So using IMAP your incoming server settings are

Username - <accountname>+<mailboxname>
Server - imap.plus.net
Password - <mailbox password>
Security - None
Outgoing server settings
Server - relay.plus.net
Port - 25
Security - None
Require sign in - checked
Username - <accountname>
Password - <account password>
Hope that helps, any queries post back.

You are a star it worked must have been the IMAP i was setting it up with POP3
Cheers

Glad to be of help. It should work with POP3 as well but the problem with POP3 is that it normally downloads emails and deletes them from the server. Typically that's not what you want to do with a mobile, you want them left there so you can also download them from another ( e.g your home ) PC. Most implementations of POP3 clients have an option to leave messages on the server, the Android client doesnt seem to have that option.
IMAP doesn't have that problem, although it might delete from the server if you delete the message on the phone, check it and see.