Hi Guys,
Apologies on the delay and thanks for the responses.
Yep, the issue was that Moodle is triggering the default configuration of mod_evasive for Apache. mod_evasive is a DoS Protection module.
Relaxing the configuration slightly has stopped the 403's from occurring.
For those interested, the following is our configuration, inside /etc/apache2/mods-enabled/evasive.conf
<IfModule mod_evasive20.c>
DOSHashTableSize 5000
DOSPageCount 10
DOSSiteCount 100
DOSPageInterval 1
DOSSiteInterval 1
DOSBlockingPeriod 10
DOSEmailNotify email@address.com
DOSSystemCommand whatever_command_you_want
DOSLogDir "/var/log/mod_evasive"
</IfModule>
Hope this helps?
Cheers,
Ray