I live in a British late 1900s semi detached house that I assume would have had floorboards originally. However it seems that these have been replaced by Chipboard. This doesnt seem very sturdy and I was going to replace. What would be the best type of wood to use to replace the old chipboard?
Sub floor material can depend on what type of floor is going to be installed on top of it, and the joist spacing.
Typically if the joists are 16" on center (OC) or less, you can use 5/8" tongue and groove plywood glued and screwed to the joists. larger joist spacing calls for 3/4" tongue and groove plywood, again glued and screwed to the joists.
If the flooring material of choice is a heavy tile, you'll want to use 3/4" tongue and groove plywood even with 16" OC joists.
Depending on what type of floor covering you've chosen, an underlayment may also be necessary.
If you are planning on installing carpet, no extra materials are required.
If you plan to install hardwood flooring, you can use felt or rosin paper as an underlayment (though not all installers use it).
If you are installing laminate flooring, you should use a foam underlayment.
If you are going to install tile, you'll want to use a concrete board or similar material on top of the sub floor.
If you want to install vinyl or fiberglass sheeting, you should use 1/4" Lauan as an underlayment.