Dear All,
My apologies for being off-line for so long; I've had to fix the gears on my bicycle.
Many thanks indeed for the bounteous advice; I have taken it all onboard. At the last shout I was able to access the Plusnet Webmail using username+wifesname, but not via OE using the same Incoming Server account name. The OE email set-up wizard had lead me to setting up another account, which failed every time I did a send/receive on it. What I have now done is deleted this new account and instead modified the existing username account in the way that you have advised. Happily, it now works fine, thank you, and I have been able to recover all my wife's missing emails.
Yours gratefully,
Stuart