Thinking about a glass plate, my heated bed appears to have some warp to it as there is a high point in the middle - can level all corners and edges nicely but right near the middle there appears to be a bump.

Am I right in thinking that Stove Glass is also going to do the job? Is far more readily available, stuff from Schott Robax is rated to 560 C for constant heating - even more for spikes in temp and can get it cut at most glass places or stoves shops to size.