My lad does this work. His spec agrees pretty much with POC's as a minimum. Basically, he digs down 1 foot (300mm) as a starter and a lot of his work is re-laying existing drives that have been poorly layed with inadequate foundations by gypos and builders. He says avoid builders doing it - they think they can do anything but there's a lot of skill and experience laying a block drive, especially when it comes to the planning and details.
Also, the biggest cost difference is in the blocks - anything from under a tenner to £30 per m2.
For a total job, he uses a yardstick cost of £50 per m2, but each job is different and this can go up, or down, according to layout, underground pipes, finishing details etc.
Hope this helps.
Richard.