[exim] trouble with redirect router and permissions

I had an account running uucp and it worked fine.
Now I've switched it to using fetchmail via tcp/ip
and I've got permission problems which I cannot understand.

my exim binary is definitely running as SUID root,
however is appears when exim gets the delivery from fetchmail
that it is running as the mail user.

I get an error message like:
"unable to set gid=6 or uid=4005 (euid=25): userforward route (recipient is mitvs@localhost)"

my mail user is ID 25
mail group is 6
the user mitvs is ID 4005

I set "group = mail" in the redirect router thinking it may fix
the problem, but the above message didn't change before or after.

I'm not sure why the (euid=25) is in there.

Can someone offer a suggestion please.