How do I get flexible email headers and sigs?

[Tony <uctpjac@ucl.ac.uk>]

Hi,

Has anyone figured a nice way of doing the following:

I have several "identities" I'd like my mail to go out under - 
tony.curzonprice@ucl.ac.uk for my academic work, tony@autopd.com for the 
start-up I sometimes act for, tony@curzon.org for personal stuff etc.

I'd like the From:, Reply-To: fields and the signature to all change depending 
on which identity I'm mailing as. The identity should default to the right one 
if I'm replying, and otherwise should be input in some quick way.

I currently use mh and manually edit things. There must be a better solution??

Thanks

Tony