I actually did some research into this for something recently, different subject (vehicles) but the "fit for purpose" bit applies to more or less anything. IIRC I found that the generic European law says any latent defects are the manufacturers responsibility for a minimum of 2 years, but in the UK for some reason it is extended to 6 years. This boat is just inside that (supplied late 2000, not sure when exactly it was built but earlier that year I imagine) but more to the point it has a mere 303 hours operating time on it from new (I checked today on the engine hour meter) and since I expect to be doing 100 odd hours a year, if the worst comes to the worst and I had to re-tube it (which is beyond the capability of me or anybody else round here anyway), I bl&ddy well wouldn't expect to have to do it again in another 3 years because the @&5e fell out of the tubes again.
Can't see me getting too far with waving the law at them anyway, they would probably say "no problem send it back to us and we will have a look" -- freight both ways would end up costing nigh on what the boat did... just as cheap to throw it away but if it comes to that it will be my first and last boat.
Not sure if the previous owner knew about it or not, the thought crossed my mind, but there is no sign of any repair work ever having been done to this area and I suspect the damage was almost invisible until it blew out yesterday.
Having been looking at everything very closely while working on it today, I am now quite sure it is a defect in the design/construction because if you look at the tube on the other side, there is a discolouration and slight swelling/stretching in the hypalon in exactly the same place - the narrow zone where a single layer is between two double layers - and once you can see that marking, you can follow it right the way round the circumference of both tubes. I guess it has failed at the bottom because as others have said that is the bit that takes the battering, but you can see the marking right the way round the tube in good sunlight. Even if I had noticed this before I wouldn't have paid any attention to it, but having seen the failure, and flexed the deflated tube I have been working on, you can see that this narrow zone is what will take ALL the flexing in this area. And you can also see that if they had bothered to overlap the hypalon by an extra inch on each side it would have been a triple layer instead of a single layer and the problem wouldn't have ever happened
Anybody else with a Humber would be well advised to check for this problem. If I can get decent light on it tomorrow I will try and take a very close up photo of the tube on the other side to show what it looks like.
Thanks to everybody that has contributed so far, my experiences are not developing into what I hoped boat ownership was going to be like but it is good to know there is a resource like this forum and the knowledgable and helpful folk that inhabit it, to give advice