I have taken the comments listed here on board and have already taken action to implement the following.
1/ Amended SPF record so that only the primary mail server is authorised to send mail.
2/ Amended the reverse DNS infomation for mail server IP address to a non adsl one.
3/ Amended hostname of mail server so that it does not display adsl
4/ Made sure that that the A Name MX record, SMTP greeting / hostname and reverse DNS for IP all match.
Although the main website is hosted with hostgator.com they do not host our email, and to be honest their service as a host have been second to none, just a shame that their backbone provider has an allegedly obvious dubious record with regards to spam.
Many thanks for all your help and comments.
Hospital Radio Haslar