In a rare moment of enlightenment I may have discovered something that was on the system causing the problem for people. Hopefully it’s the same solution for everyone!
In this day and age, being a webmaster for a website involves more ‘skill’ in keeping the bad guys out whilst keeping the site available for the good guys. Every system that has PHP installed is vulnerable to a multitude of attacks from those based on PHP vulnerabilities to those based on attacks against the actual scripts themselves. To overcome this (and many other security issues involved with web servers) I have to run various security measures to try to stay one step ahead.
Unfortunately in this case, I was probably about 10 steps ahead and had left behind legitimate users too. I have tweaked the settings and, given that DD was the only site on the server that was receiving complaints about not being able to get in I think it may be correct.
Please let me know if you believe the problem is now resolved – if you still experience problems of a similar nature I will go ahead and look deeper at the security systems.