Well… we have a mystery on our hands but one that may never get solved!!
So far, everything has remained working as it should! Which is great, and thanks for the help/pushing me forward to taking another look.
The mystery part is this:
I had this at the top of the rule file:
rule "Pre-Load Lighting Rules"
when
System started
then
logInfo("Pre-Load Lighting Rules", "finished")
end
as well as the additional RFDataLights check in the if statement.
So as I said in my last post, I had removed the extra RFDataLights check. When I would reboot the Openhab server, if the lighting group was switched OFF, it would turn them ON when Openhab started… However, if the lights were ON, it wouldnt turn them OFF when Openhab started… very strange!!
So I removed the pre-load of rules and seemingly… all started acting as it should.
Since then, as I say, all has been fine!! 
For the sake of completeness… just to prove things… I put the rules, bar the pre-load, back to having the additional RFDataLights check in the if statement… and… well… everything still worked fine.
A bit more playing around… I dont exactly know what caused the problem, but it seems to be something that is a mix between the Pre-load of rules AND the additional check that causes this… but not either of those solely on their own. Thats the best I can ascertain anyway… And yes, I did have some other Pre-load of rules in some other rules files, of which they are all cleared out now.
So I think it will remain a mystery as to how all these things were interacting as they were… but my system seems to be all happy and stable again now… Though I thought it worth leaving this here, should this strange occurrence pop up for anyone else in future.
@rossko57 Thanks for your help!!!