I wondered that myself about the best before date and exactly how old the product I had was. None of the concentrates I got from Flavourworld had a best before end date, and also one of the repackaged and rebranded Inawera flavours (and I forget which) was more like 7ml that the 10ml I paid for. All a bit hit and miss I think I'll stick to ordering from main suppliers in future (he says, until the next sale comes along)
I mixed some at 6% a while back and I had to dilute it back down to 3% it was vile at 6%.
Interestingly, for me at least, @SouthCoastVaper can tell the future, couple of days after my original post I am sneezing, snotting and coughing and have what seems like a chest infection brewing. (should've ordered some menthol)