On the basis of what you say, the construction/design of the grates is defective. If the EC has that kind of attitude, then perhaps you need to up the ante slightly.
If they don’t agree to do it, say to them that they actually have an obligation to fix it under section 62, and if they do not fix it you will make an application to the CTTT for an order to compel them to do so.
The fact that they put carpet strips under the grates means that they have recognised there is a problem, and that helps your case.