I Think I have found the problem. A few weeks ago I installed a mod for the registration page. The function on this modification was to ask the new member a question and there is a box for them to type in the answer. This just makes sure that they are human and not a bot trying to auto signup as we were getting over 100 sign ups aday and 98 of them are fake.
I think that the mod allowed hackers to do a "SQL Injection"
I have now removed it so it should now be OK.
I havent upgraded the forum to the latest version yet but I will very shortly incase that helps with security.
I am taking regular backups etc so no matter what happends now we can be back up and running within an hour from when I realise anything is wrong.
Things happening like this just makes us better prepared in the future
All the best guys. LAz