The Sale of Goods Act states that a product must also be "as described" and "of satisfactory quality", as well as "fit for purpose"..We have to accept that although we do have Consumer Rights in the form of the Sale of Goods Act, stating that an item has to be fit for purpose. It would be debatable as to the 'purpose' of e-cigs in their current form.
So if an item is DOA or in any other way faulty on receipt, it won't be any of the above. As such, one legally has the right to "reject" it and request a full refund. The period in which one can do this is normally not very long and will vary from supplier to supplier, some only give 7 days in which to claim a refund for faulty item - not very long in my book, but there we go. This isn't the same as a guarantee which won't necessarily offer a refund but a replacement (or repair) only.
A guarantee implies that the product should last the duration of the guarantee. That being the case, then doesn't the issue become more one of being "not as described" rather than one of not being "fit for purpose"? In which case shouldn't suppliers not make claims that the items last longer than their guarantee?