Initial inspection of the floor finds that the front left hand corner and side (where it meets the side wall) is both rotten and de-laminated. I made a start on removing the top plywood skin from the floor.
Where the plywood was soft i scraped this away with sharp wood chisel being careful to keep the chisel flat in an attempt to not damage the foam core. This was a slow process so i decided on a different angle of attack.
I marked the floor using a straight edge and a permanent marker, then using a Dremel that had a depth guide attached (set to 5mm) and a spiral cutting bit installed i was able to follow the lines marked out on the floor. Then using a wood chisel i was then able to prise away the plywood skin.
This worked perfectly! by limiting the depth of the cut to 5mm only the plywood skin was cut with no damage to the underlying foam core.