You may, at the nd of the day, require some new chipboard bits (either 18 or 22mm usually) if the hatches you will create get damaged.
You'll need a drill and a jig saw, and some lengths of 2" x 1" timber as bearers.
Look at the run of the flooring joists - you'll get some idea from the nails / screws pattern. Start by drawing a line perpendicular across where you want to be, and drill a series of 3mm holes, working the drill to and fro until you can get a jigsaw bit in. Then use the jigsaw in that slot until it contacts the joist one way, and then in the other direction until it hits the opposite joist. At that point, do the same distance away, so you have two perpendicular cuts. Draw a line to form a rectangle now, and use the same method to cut out that rectangular patch.
DISCLAIMER!! No one here knows where your central heating pipes are - there's always a risk when cutting boards! If I have doubts, I grind a jigsaw blade short so that it cuts through the floorboard, but no more.
By using small drillings, the chipboard patch can be reused, by securing the 2x1 timbers to the underside of the existing chipboard, and fixing others on the side of the joists - just so the replacement patch doesn't fall through.
If you want to be less precise, you can drill bigger holes, but you'll need to renew that piece of chipboard if you do.
Hope thats reasonably clear!
John
