Newbie asks MORE decking questions !

Hi guys ( and girls )
After much reading, drawing , thinking and talking, I've finally ordered a whole load of decking bits and bobs arriving this weekend. Without boring anyone to tears, i'm building 2 decks of 4.8 x 2.1M, joining at 90 deg on the end ( so one 'leg' is 6.9 x 2.1 ). Most of it is on the ground, with a slight ( 12" ) fall max at one end.
I'm fine with the posts/supports/bolts etc, but have a couple of 'nigglies' before I start -
In getting rid of years of vegetation, the ground is freshly dug over. For the sake of 25 notes should I hire a 'whacker' plate to really flatten it ?
The drainage fall - Long story, but it has to go along the 6.9M length. Following the 10mil in a M rule, thats nearly 3" ! will half of this do ?
Noggins. The timber merchant assures me I dont need any. As the 4.8 x 2.1 has the joists running the 4.8 way, all 420 centres, with all the 2.1 boards running across these, is he right ?
Sorry its a long note, but I really value the opinion of people who have done it, rather than those who just sell it ! Ballustrading and paving to follow............

No whacker.

You can have a smaller slope. Just make sure your deck boards dont dip between joists.

Have a few noggins. Dont overdo them.

If joining deck boards end-on, dont try to attach the two ends into the same 2" wide joist. Nail an offcut onto the joist for the second board to be screwed to.

Since you have recently dug up the ground where the decking is to go, I'd suggest putting down landscape fabric to stall any weed growth under the deck.

Thanks for the replies guys - Good idea on the 'double joist' for the board ends butting up. Glad I joined this site; the related question from someone else about Posts being better than newels is good, as is the idea of turning the boards upside down and using the smaller grooves on the bottom........

we always wrap the frame in the membrane rather than putting it on the floor. stops stuff dropping through and weeds such as bindweed etc, finding their way up through