Single skin, un-insulated flue pipe must be at least three times it's diameter (OD) from combustible materials. Combustible materials must therefore not be within the grey shaded area shown in the diagram below.
For example a 125mm single skin flue pipe must be at least 3 x 125mm = 375mm from combustible materials (being pedantic the actual figure is a little higher as 125mm refers to the internal diameter not oustide diameter).
The combustible material can be heat shielded. The flue must be at least 1.5 times it's external diameter away from the heat shield, the heat shield must extend at least 1.5 times the flue's external diameter to each side of the flue, and there then has to be an air gap of at least 12mm between the shielding material and the combustible material.
Practically this can be acheived using 12mm fireboard. Strips of cut fireboard are used as spacers behind to achieve the 12mm air gap.