[Exim] domain-dependent envelope-mods

I'm an exim newbie, so please forgive any howlers.

We currently run a mail hub through which all incoming mail
enters and is forwarded to its final internal destination
via an alias list.
All outgoing mail first passes through the hub and has the
sender's domain forced to mrc-lmb.cam.ac.uk in the envelope
and appropriate message headers.
This is currently done with sendmail :(.

I'm about to switch to exim4, and whilst I think I
can reproduce the above actions OK, I would like to
add some capability so that we can support a second
mail domain (say mrc-centre.cam.ac.uk) in such a way
that for outgoing mail, the sender's domain is forced to
be either mrc-lmb... or mrc-centre... depending on the
name of the sender, which would be looked up in a file
to determine which domainname to use.

I cant work out how to make exim change the envelope
sender-domain (and the appropriate message headers)
in a way which depends upon both the sending-host
(only do this at all for outgoing mail, i.e. mail with
sending hosts *.mrc-lmb.cam.ac.uk) and which also
depends upon the sender's localpart, which would exist
in one of two (ideally alias) files, one for mrc-lmb members
and one for mrc-centre members.

Any pointers would be appreciated. Even such as 'this is
perfectly possible, just RTFM'.

Cheers,
Terry.