Size matters..
Hi Phil, well your sorted for a bag, but I would have said..
In general terms.
35ltr - if you are solo and can pack neatly and not heading anywhere where you need lots of cold weather gear, ie fleeces etc..
49ltr- if you are solo or two up and can again pack well and dont need too much cold weather gear.
89ltr - if your two up and or solo with cold weather gear, or want more space for bringing shopping/momento's back.
As for packing.. I personally carry my tent and sleeping bag in my bag, and my cloths, food, cooking gear etc in my panniers. I know one school of thought says to ensure your tent and sleeping bag are inside your panniers, for security so you should your bag go missing, you have your shelter, but for myself I work on the theory that the items I want to unload first from my bike when I get to a campsite in bad weather, is my tent, then my bedding and then, change of cloths, then food etc. I always pack a torch where its easy to get hold of too, as arriving in the dark in the uk is a occupational hazard..
Paul, Stubbs comment is about right, if your tent takes up the major space in the bag, then put the stuff you want to keep dry in a dry bag.
Oh for those who havent seen em yet, exped are presently doing square profiled dry bags with air valves in them, so you can compress them;
See Here and
See Here
Great fit for TT and BMW panniers. Also makes packing a hell of a lot easier.. stuff your compressable cloths in with open valve, close bag, kneel on or compress bag, close valve, adjust roll closure if you wish and whalla.
Exped also do some roll tops with clear sight panels in the sides so you can actually easily see the the contents. Usefull if your not a greatly organised packer like myself and don't want to have to haul gear out to find a specific bit of clothing etc.
Phil, with the roll bag you have gone for, can probably pop your tent in it and some clothing (rec a seperate lightweight bag), tent last mind. Though anyone who puts a tent in a bag, seriously recommend you always roll your tent poles into the middle of your tent when packing it up, and not put them in last, as when you strap the bag down, its easy to be a bit heavy handed and wind up with bent poles.