The conventional method is to use chipboard, however I don't like the idea of losing access to the void without pulling up the whole floor.
In a victorian house the floorboards were 7" x 1" PAR timber which made access a dream.
7" x 1" seems very expensive, and more expensive per sq.m than decking which will also have been treated.Are there affordable sources of 7" x 1" or should I just go for decking!!??