qwertee:
Thanks for your feedback.
Yes its an internal garage.
I am over-thinking the whole thing and with the advise from others my plan is to:
1. get plywood sheets (12mm) and screw them straight to the rafters around areas where there are supporting walls and store stuff
2. install attic stairs later (when I have enough $$) using the existing trap door
cheers
Also while you are at it look at some form draught sealing on your internal access door,
it looks like there is a healthy gap under it....
(the garage will be a huge heat loss in winter as it has a lower insulation rating than the house)