This is all great advice thank you everyone.

My conclusion is it should be ok as long as I'm sensible. I've got 4 ROK straps and the velcro on the bottom of the bag is huge. If I can get that secured under the seat then I think it would really help. I've never heard of these webbing loops under the seat so I will get out and look for them. I've been strapping it to the rear foot pegs and some metal that the license plate holder and indicators are attached to, directly under the pillion seat.

There's no chance of putting this bag on the bike without it touching my back anyway. At least I'll know if it's starting to shift or I can wave goodbye to it when it comes off. It hasn't moved an inch in London but the big boy speeds out of the city are what I worry about. If the bike's more stable at higher speeds and 4 ROK straps should do the trick then that's perfect.

70l is definitely on the big side. I'm not really sure what I was thinking buying a bag like that for a skinny little streetfighter but it works great for my commute, saving me having to carry any bags at all the rest of the week.

I like having a single bag, it's a dead simple setup. It's roomy enough for a RTW trip as far as I can tell. As a test I loaded it up with my tent, sleeping bag, two sleeping mats (one of them being one of those old school closed cell foam ones), a bunch of clothes, my hiking boots, rain gear, and it was loosely packed. I still need to work out my electronics and tools since I don't actually know what I need but if it's secure enough then I'm dead happy.

Thanks again all