Hi Marcha
I tried using the IMAP settings, but that didn't make any difference. However, I did try using the K-9 mail app instead of the default app & that did allow me to send from all accounts, except for the TalkTalk account. Same settings, but a different app — weird! Just need to raise the issue with TalkTalk now, as the problem seems to be at their end, rather than with the phone or Orange. Not sure how successful that'll be!
Russell