Trouble with forwarding configuration

[A.Cornah]

I am trying to set up sendmail.cf files so that departments like my own
can hide behind a routing JANET connection, forwarding everything to be
redirected if it is not local.
I have set up three machines here to simulate this first:
Department router world
cluster server sunc
When I
mail to...@server.shef.ac.uk
from server
then the message arrives OK on server from "to...@uk.ac.sheffield.cluster"
BUT
when I
mail to...@sunc.shef.ac.uk
from cluster (from server 5 lines above should also be from cluster)
then the message arrives OK on sunc BUT it is from "tony@cluster"
Have you any idea what I am likely to be doing wrong?

Also I can't reply to the message I described above as going from cluster
to server
Any ideas again?

Thanks
tony cornah