Thanks everyone for the advice, comments and support.
I'm still not getting forum notifications - so never know when to come back to read and reply!
I received a subscription renewal notice from the Developer this week - to which I responded quite firmly. I did get a reply, saying sorry I felt they had a negative tone and weren't providing support. Then saying when I told them the problem they did respond quickly.
Although I use their modules a bit, I won't (and can't) continue paying a subscription after this so I will just leave the latest versions as is and look for other or custom solutions to replace these in the future. Unfortunately I'm not a developer and I have limited human resources available here that can help.
I did have a test environment running before (same server but different DNN instance and database) - but when this happened it was scrapped to clear up as much as possible in attempts to get it working and identify the problem.
Had been taking back-ups, though some of these failed before one guy cleared it... bearing in mind we couldn't identify this module as being the culprit so going through backups we weren't sure how far back to go.
The bit that surprises me, and I do have to agree backups and a test environment are key items to running smoothly, is this spat up no errors from within DNN, killed .NET2 (both test & live environments) and the timing of it happening didn't show what it could have been. How does one truly protect against that? Of course we don't have any real programmers/developers here but I sought advice, provided details and then proceeded to contract 3 MCSE/MCP/Developers before someone could start to narrow down the problem (and had no knowledge of DNN at the time).
Overall I just think it was a very unfortunately position, and something people using DNN should be aware can happen. It did cost me a lot in money, lost income, reputation and most of all time (3 weeks down and then recovery which is almost done now - technically).
After going through that, one would hope the developer would show a little more interest than very short support emails asking what version and suggesting installation of the latest version.
Many thanks again.
David