The hosting company requested that I take the server down and whitelist it in case they need to restart it etc so that they can check some things out, as Friday mornings are generally quiet and I didn't want to disrupt the weekend I've taken it down now.
Sorry for the inconvenience, I'll bring the server back up and remove the whitelist as soon as they give me the go ahead.
An update will be posted here asap!
UPDATE: 11:30 - Still waiting to hear from hosting company.
UPDATE: 14:20 - Server is back up and whitelist removed, however unfortunately they have still not got to the bottom of the disconnect issues.