jimgriff,
You should reallly sort out an A record and point your MX to that.
;; QUESTION SECTION:
;aberystwyth.gov.uk. IN MX
;; ANSWER SECTION:
aberystwyth.gov.uk. 3600 IN MX 5 213.171.216.114.
What might possibly also need to be addressed is your reverse record which should really match the hostname provided by your server in its HELO, which should also match the A record that you will define in response to the previous point. Otherwise some strict anti-spam software might filter you out.
;; QUESTION SECTION:
;114.216.171.213.in-addr.arpa. IN PTR
;; ANSWER SECTION:
114.216.171.213.in-addr.arpa. 86400 IN PTR smtp-in-114.livemail.co.uk.
I also see that your IP address is listed on 3 anti-spam blacklists, which might be a contributory matter to your problem
At the time of writing, these were :
Backscatter.org
SORBS
UCEPROTECTL