I used my normal Camelbak Hawg (13litre volume, 3L bladder) with a sleeping bag attached to the outside for a bivvy on the moors last june. While it worked very well then, letting me ride the round hamsterley black with no bother, I know its not the best solution to some more adventourous mutliday type stuff I’d like to have a go at.
I’ve nothing planned at the moment, but some bivvying (with small-packing sleeping bag), maybe multiday yha-ing (with change of clothes etc) and what not I’d like to have a go at.

I’ve not had an extensive look, (just on camelbaks website) and the following two look the part:

Under the biking part of the site:
Camelbak Hoss, 23L volume, 3L bladder, RRP = £110, available for £87.50

http://www.merlincycles.co.uk/?fn=product&productId=1322&categoryId=47

Under the running part of the site:
Camelbak Octane 14+, 15L, expanding out to 23L volume, 3L bladder, RRP =
£90, available for £71.34

http://www.wiggle.co.uk/p/Cycle/7/Camelbak_Octane_14_Plus_3_Litre_Hydration_System/5360028104/

Though the Hoss has the exterior stuff sacks, the straps look pretty unstubtantial

The Octane on the other hand looks to have much more secure waistbelt and the flared waist pockets/straps look like they would be massively more secure on your back. Theres no exterior stuff sack though for all the usual bike cr*p that lives in there. The side pockets look very useful for a camera, food and other bits and bobs though.

Does anyone have any experience of either of these two, or reccomendations of any similar bags please?

Are these just way too small for any ‘proper’ adventures. Is it time to face upto a singlewheel trailer like an extrawheel trailer?
http://www.extrawheel.com/

Consider a Wingnut. There was a frankly fantastic post from a guy that did a tour of Scotland with one, and it seemed sufficient for his needs.

Personally I use a Vaude pack re-badged as Edinburgh Bikes, it fits 25 to 30l I believe but unfortunately there’s no webbing for kit to be lashed to.

I love my wingnut and can heartily reccomend it.

Kimm/Omm 35rL.

a-mazing bag

As bigrich said OOM sacs are great – I had a Wingnut but didn’t get on with it