You should receive two warranties to cover you. One from the manufacturers and one from your original installer covering parts and labor. That warranty should have been insurance backed as cover in the event of your original installer ceasing to trade.
I don’t believe a national companies will give you the same aftersales service as a local company will. Nationals will be the first to disappear once the incentives are gone.
I always say every company will charge differently, depending on the amount of staff and overheads of the company. A one man band (to coin a phrase) can install cheaper than a company with half a dozen staff.