I recently had some windows replaced by a double-glazing firm. Now the bricks above the windows have started to sag because the new units can’t take the weight. The window firm has offered to put in lintels across the top of the windows as long as I pay for the materials, but I think their surveyor should have spotted the problem in the first place.
I think you are right. A firm that employs a surveyor to assess the suitability of its products for your house must be responsible if it turns out they are unsuitable. You should suggest very strongly that the window firm carries out the work at its own expense. On the other hand the cost of a few lintels may not amount to much, and you may be risking extensive damage to your property while you wrangle over responsibility.