The hearth should be of a certain thickness and be constructed of non combustable material, can't remember the excact dimension (building regs). The Logburner instructions will give the dimensions for the sides and back, the top of the burner could reach about 400 deg c
again there are building regs covering distance of combustible materials from the flue. Some chippy's boxed in and made a chimney breast after a fitter had signed off a chimney and appliance and burnt the barn conversion down within months
Shefield insulations do fireboard, I use it where I can when I solder with map gas as it lasts much longer than most heat mats. You can use this stuff as a closure plate between the chimney and the stove pipe
