You said that Approved Document P indicated something which does not appear in the legislation (Part P of the Building Regs) that the document claims to be 'clarifying'. I therefore suggested that you should probably ask the author of the document ("Mr Approved Document P") where it came from or, perhaps, you could ask the Deputy Prime Minister - since, at least in the past, Approved Docs were published by "The Office of the Deputy Prime Minister"!!Sorry, may be a bit brain-fugged tonight (most nights?) but I can't understand what your sentence means...You probably should ask Mr Approved Doc P (or maybe the Deputy Prime Minister) where in the legislation he found that one!!
Kind Regards, John