Just a quick update to this post. We had the same seat with the same problem. It turned out to be a bottom fix seat fitted before the toilet was installed. The solution was to prise the seat off with a screwdriver on one side ( wrap the screwdriver in j cloth to prevent scratching the porcelain) and then use a hack saw to saw off the bolts. Debris removed and the toilet readily took a new top fix seat. Hope this helps someone else.