New roof guarantee breached

Wilcote replaced our roof, guaranteed it for 8 years.After 5years they are refusing to replace rusted sheets .Firstly they ignored our calls and emails for months,then claimed that they hadn't replaced the roof so it wasn't part of the guarantee and then blamed their supplier and have agreed to replace one rusted sheet on condition that we nullify their guarantee and make no further claims.We have tried to contact Wilcote Headoffice but have had no reply.We saved up for years to replace our roof,believing that we were using a trustworthy, reputable franchised company with a Headoffice that would ensure that their guaranteed standards would be upheld .We are simply devastated by the way that we have been treated by Wilcote.