It was the OnyakT. NukeAl*rt module that had killed my site... a module I'd run with few problems for nearly a year... so an update - and with the way the error appeared, it wasn't even clear it was something in DNN, let alone which module it might have been.
I have been happy with the modules they provide, and from what I've seen and heard I felt they had a good reputation also. I am not sure what it is with this module, and how the problem happened but I am not running the latest now as I am a bit exhausted from the problem previously - apart from that everything is pretty much back to normal and no other changes (everything else is the latest etc - except DNN since it has moved on 2 versions for security.
I do take backups regularly, but for some reason there was a mess made between the back-ups, trying to run DNN with minimal things, clearing up anything and everything else on the server, and wiping and starting from scratch (then not knowing which DNN backup to use anyway).
It just created a mess out of a few weeks - so even though I am happy otherwise with the modules I will not be renewing my subscription and will look at other options after receiving a subscription renewal notice... it's just hard to want to send more money that way after what happened.
Brandon was great too... offering suggestions and to take a look outside the forums. Thanks for that Brandon!