I am trying to get down to just a bar mounted dry bag (13l) and a large seatpack and no rucksack for the WRT. Bottles, pump, tools, spares are frame or fork mounted.

Can decide what is best packed on the bars other than the sleeping bag? All the other sleeping gear? Bivvy bag, mat, tarp? Though this means packing potentially wet things in with the sleeping bag I need to keep dry. Or do I put camp clothes and extra riding clothes in the bar bag and keep it to just things that must stay dry then put everything else in the seatpack which probably won't be 100% waterproof.

Have bivvyed before but only on foot never from the bike. I might well be obsessing over this too much

Or do I put camp clothes and extra riding clothes in the bar bag and keep it to just things that must stay dry then put everything else in the seatpack which probably won't be 100% waterproof.

You've answered your own question

For me its dry bag at the front where things need to stay dry. (sleeping bag, dry/sleeping clothes) Generally stuff on the front doesnt get too wet from puddles etc.

Anything on the back of the bike has to be able to cope with getting damp, so bivi, tarp etc

Roll sleeping bag up inside bivvy bag, put on bars. If you can get your sleeping mat in there too then even better, instant bed.

If you can get clothers, food, stove, and tarp in seat pack you're laughing, but maybe a tangle/frame bag will make it easier.

Where I pack stuff depends on which bike I'm riding but I still try to pack stuff the same way.

1/ Sleeping bag, down jacket, hat + bivvy bag in seperate dry bag in case it gets damp.
2/ Tarp, pegs and groundsheet.
3/ Cooking gear and food.

The above are kept apart in seperate dry bags. If I need to carry any other 'little bits' then I'll use a 6l bum bag too ... a very handy and underated bit of kit IMO.

Tools/spares just strapped wherever there's room.

Thanks for the input, need to get everything together and try packing. A bumbag is something I hadn't considered but might be great for Day food, first aid kit, phone, thin extra layer, etc

Agree s8tannorm, I use an ancient bum bag (from GAP at least 10 years ago). The most useful bit of gear I own - strapped round a bag on the bars it's a 'general purpose' holder for GPS, food, beanie, buff, spare gloves , glasses, phone (endless list of little bits..), around camp, keeps pockets empty so stuff doesn't fall out and, if I ever have to abandon the bike & 'walk out' for whatever reason, makes the perfect emergency pack..

http://www.salomon.com/uk/product/custo ... ocket.html

just got one of these, quite handy and for only about 25quid

Good call on the bumbag, will be copying that great idea :D
I generally put everything in the front bag required to make camp, so that bag only gets opened at the end of the day. All clothes in the rear.