A Guarantee can mean nothing unless you really back it up.
We buy direct from all our suppliers and part of our agreement with them is that we honour the guarantees that the manufacturers give and that we perform to a high standard after all we use their company name in all our literature and on this website. I think it is fair to say that you should make sure that whoever is carrying out the work for you that they have not just bought your loft ladder or roof window from a DIY store and then purport that you have five or ten year warranty because unless the installer honours the guarantee you will not have a leg to stand on if anything goes wrong.
The guarantee on the workmanship that is carried out should also be in writing and should be at least one year on top of the product warranty. Every workman or company that is carrying out work at your property should have product and public liability to cover you, your property and its fixtures should anything go wrong and that is a completely different subject for another day