Can the dimension of 400mm from each side be reduced if a non-combustible material is used, such as a fireline board ?

Hi, thanks for your enquiry.  You would have to use an A1 rated fire resistant board, this cannot be stuck onto a combustible surface as there needs to be a 25mm air gap around the fire-resistant board (bottom sides and back). Fireline board is usually A2 rated therefore this doesnt meet the hetas regulations and wouldn't be suitable.