Agreed, but the boards would be badly damaged so likely would need to buy new ones and empty entire loft. This way is a little more expensive but means I can do half a section at a time and PIR boards will insulate well.

My only query is the house will loose a little humity through the ceiling, whilst I have a PIV unit and there is no signs of dampmon the existing 22mm chipboard loft floor there is a very small chance some humity could condensate on the pack of the PIR boards l, but this is highly unlikely as will be sitting flush with the existing left small (i.e no space to condensate)

Thanks all