I got 3 replies and have managed to solve the problem.
Thanks to uunet!etnibsd!vsh (Steve Harris) who suggested I tweak the
sendmail.cf to suppress the PC names.
Here are the relevant lines to change:-
---------------------------------------------------------------------
# Convert uucp sender (From) field
S13
R$+ $:$>5$1 convert to old style
R$%y!$+ $2 strip local hosts <<< CHANGE HERE
# R$=w!$+ $2 strip local name <<< USED TO BE
R$+ $:$m!$1 use domain name for host
This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:06:18 CDT