For anyone who might be reading these e-mails some time down the track, here is the solution I finally came up with
begin authenticators
plain_login:
driver = plaintext
public_name = PLAIN
client_send = ^username^password
server_prompts = Username:: : Password::
server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}
server_condition = ${lookup{$1}lsearch{/etc/exim/passwd.client}{${if eq {$value }{$2}{yes}{no}}}}{no}}
server_set_id = $1
However for the LOGIN authenticator use:
client_send = : username : password
Thanks for everyones help it was much appreciated
de Chris G