Sounds like an internal DNS issue.
From a workstation in the internal network, what reponses do you get with doing a nslookup for moodle24.olchs.lancs.sch.uk (the fully qualified domain name - internet - not Windows)? If you see the public IP address (external) then workstations inside the priviate network are basically attempting to loop your entities firewall to access.
Outside DNS does resolve to an IP address.
In order for moodle to be seen both inside and outside a private network there has to be a DNS entry in both external DNS (which there appears to be) and a DNS entry in internal DNS. In your internal DNS servers add both a forward and reverse entry for moodle24.olchs.lancs.sch.uk that resolves to the private IP address.
Since content has been developed with a Windows internal network/shortcut name and not a FQDN, images and some links to files may not work after making the change to DNS and config.php. There is a tool to Search and Replace in Moodle 24.
Search for http://moodle.moodle24/
Replace with http://moodle24.olchs.lancs.sch.uk/moodle24/
Include the trailing slashes.
'spirit of sharing', Ken