I've emailed Avon, as yet i've had no reply from them, if no one else can help i'll give them a call, the hatch is a longer term project but just thinking forward rather than just assuming i can just cut a hole in the deck.
The flooding hull has been blanked off at both ends and i pretty much intend to keep it this way, for my needs i dont need to flood the hull, the boat handles and sits just great without flooding it. Of course though the option is still there if in the future i see a need to flood it.
Some one suggested marking the diametre of the hatch out on the fordeck where i want it to go and drilling a hole through, then using a bent lenght of wire with the bend exactly half the diametre of the intended hole, poke this through the hole and turn it around and see if it makes contact with anything, if it does not i can assume its safe to cut the hole, if however it does contact something i need to move the hole slightly up/down or right/left, then once i've found the space remark out the diametre of the hatch to include the holes drilled through and cut the hole out......! all makes sense to me and i suppose if the worst happened and i had to abandon the idea a small drilled hole would be a lot easier to fill with a screw and some sikaflex than a big gapping hole in the deck.
Ok one other big problem would be if i decided to use the flooding hull again....!