Lifetime guarantee. So what does that mean, exactly?

When my Kohler Daryl shower developed a fault I was asked to pay for the repairI had two en suite showers installed by Kohler Daryl in 2009 with a “lifetime guarantee” for each one. The metal fixings for the sliding door on one of the units have corroded and broken. When I asked for a repair, I was told Daryl products ceased to be covered under the “lifetime guarantee” when the company was taken over by Kohler in 2005. My guarantees were issued four years after this, but