[Exim] hiding alias names

I'm fairly new to exim, and currently wrestling with an issue which, at
first, I fully assumed would be one of the oldest of chestnuts. However,
after much rummaging through the FAQ, the mailing list archive, and
Usenet, I've not been able to find anything appropriate, so I've decided
to ask the question: profuse apologies if it is an oldie, and I'm
missing something obvious.

Assume that I've got a mail server mydomain.co.uk, and an entry in
/etc/aliases:

dodgy_suppliers: fred.bloggs@???, joe.doe@???

If I send a message to "dodgy_suppliers", it is sent successfully at the
expanded addresses, but arrives addressed to
"dodgy_suppliers@???", not the expanded address to which it
has been sent.

For blatantly obvious reasons, I don't want the users collected under
this alias to become aware that I have thus categorized them!! What I
want is for the "To:" header line to contain just the current address
from the alias expansion. This seems to be an absurdly commonplace and
trivial requirement, but as yet I've not been able to find an explicit
way of achieving it. I eventually tried a rule to rewrite the "to"
header line, but at the point the rewrite happens, $local_part and
$domain still contain respectively "dodgy_suppliers" and
"mydomain.co.uk": there don't seem to be equivalent variables containing
the parts of the expanded address.

Thanks in advance for any assistance.

--
Peter Duffy