Certainly in mill type conversions I've been on there was a requirement to overboard (plywood) with areas directly above shop/retail premises sometimes requiring 12mm cement board on top of plywood to make the required fire rating (bearing in mind there was also plasterboard on the underside of the ceilings). Have they actually specified the board, and thickness, they require?
As to laying it, you'll need to clear something like half the room and lift the finished flooring on that side, then lay the fire sheet material on top of the existing sub-floor and fix it down with screws or nails, transfer your stuff to the installed side, and lastly install the fire boarding on the other side. You normally lay 8x 4ft (approx) sheets, so you need a fair bit of space to lay stuff down - it can't be done in "penny packets". Ideally you need as few joints as possible to achieve the fire rating, hence the need to lay full sheets
