I was notified today by my host that they had taken the site offline due to some malware they'd detected (for which I was grateful).
Spent the evening identifying it and removing it.
Then fully upgraded all versions to current, removed all items not contained in new version and testing. The forum should now be working and be rid of the malware too.
After investigation, I determined that the malware would have come in via a poisoned ad (supplied by google ads). Sadly this is hard to prevent. I had decided to put ads there to try to cover hosting costs (which they don't even nearly).
I will have to consider the future.
In the meantime, hopefully everything in here will keep working. Please let me know if there are any issues.