[Exim] HELO & multiple domains

I run several domains from one box. For various reasons I wont go into I can
only have 1 IP address so change of ISP is not feasible. For the most part
exim seems to work fine almost out of the box. However, whichever domain I
send mail from the helo
greeting responds with the primary_hostname & qualifying domain: so if my
primary host name is 'foo' and my qualify_domain is 'bar.com' and I send
from one of my other domains, e.g. wibble.com the helo data is 'helo
foo.bar.com' not 'foo.wibble.com' .

The mail actually get sent OK. The problem however is information leakage. All
this didnt use to be a problem since I used to use just the host name with no
qualifying name: e.g. just 'foo' but now a few remote hosts are declining me
and insisting on a fqdn, anti-spam I guess. Problem is that the above issue
causes information leakage and a customers using one of my professional
domains looked at my headers has commented on the content of other domains as
being unprofessional.

I need to solve this leakage. but I havent been able to figure out how to get
it give the right helo dependant on each domain Im sending from. I want to be
able to say something like;
'remote_smtp:
driver = smtp
helo_data = ${sender_domain)
or
helo_data = ${lookup{sender_domain}lsearch*{/etc/mail/domains/helo}{$value}}
'
but as an exim newbie I couldnt figure out the syntax either.
Anyone got any ideas? thanks