terrible service from Victoria Plumb, I needed a replacement toilet seat and they've changed all the fittings for the seat and the new one's don't fit. They were really unhelpful and won't even pay for the return postage when it's their fault ... cheap but for a reason, don't touch them
In what way does the new seat not fit? I wonder if it is the hinge that is different?
In recent years the old plastic block and tube hinge has been fading out, and a stainless or chromed hinge, often with an L-shaped bar, is more popular.
Maybe you need a new hinge to go with your new seat. They are usually supplied complete, seat and hinges, as new seats often do not fit old hinges. I have never (yet) found even a very old WC that the modern hinge will not fit.