joists no less that 16 inches apart atherwise you get a bouncy deck.
Dont bother diging holes - If the council are lifting up paving slabs locally, ask the man if u can buy some of the old ones - they are stone/pebble concrete mixes and they dont break no matter what yo do. I got 20 last year for 20p each from the man.
But them on the grass soil whatever, then make legs from your deck joists to the slabs. Make sure you put some damp course between the wood and the slab so water dosent get drawn up through the slab and into the wood.
Put the black ground cover down under the deck, to stop weeds growing.
If your going to make you deck a specific length/width, buy wood that length. Theres nothing worse than a frankenstein deck with bits all the wrong length trying to match up.
Dont make the gaps too big - mine was an M10 bolt apart (it was the only thing I could find that I had loads of that we could use to guarantee an even gap over 6 meters - so we put one every 50cm and butted each deckboard against them) use something smaller - you will be amazed at the anount of crap we lose down it
Dont go to B&Q - find an out of the way timber merchants that deliver. They are so much cheaper.
AND
Wear sunscreen if your making it soon - no need to be burned and peeling while your working.
Here's one i sorted out last month for the kids paddling pool
D