Michael Chare pretended :
It is impossible to phone Everest but I did make contact via their website
chat. They told me that the windows were installed in 2008.
and said "as the door (?) was fitted before June 2020 this would have been
installed by Everest Ltd who went into administration in June 2020 we are
Everest 2020 Ltd, a different company therefor cannot honor their guarantees
from that long ago"

Large DG companies frequently go bust and a quick name change, avoids
their liabilities. They are best avoided, best find a small local
company who will be cheaper, if you really cannot fix it yourself.