As an absolute minimum, you should allow for 25mm of sand to bed the blocks on and 150mm of well compacted Type 1 under that. This assumes that the formation level (the level you are at when you have dug out the 250mm) is solid and not cr@p. Laying the blocks is the (relatively) easy bit. Do not skimp on the preperation. Oh, and lay some Terram or similar on the formation before laying your Type 1.