You're right Lin, but I can maybe see why this has happened.
I guess you have had replacement glazing only ......... installed in existing windows?
Was it a glazing company that you had, as opposed to a upvc window company? A regular glazing Co. might not have the gaskets for all uPVC windows they come across.
I agree they should be geared up for these eventualities. Glazing is only as good as the gasket.