Hi Swallow - you're right. I got my litre and pints mixed up - this new-fangled currency and us oldies! So for litres in my post, replace with pints.
As to liquid intake, remember that its liquid - coffee, tea, squash, beer, liquid in apparently solid foodstuffs etc, not just water that gets imbibed, and subsequently leaves the body as either urine or water vapour/sweat. And in a sub-zero climate, the air is very dry and you sweat/exhale more water vapour, making rehydration critically important.
The general point however is still valid, at night you will sweat into your sleeping bag, and this will transfer body oils and dirt from your skin into the insulating lining in your sleeping bag. Even if the water vapour escapes to the outside of your bag, the dirt/oil remains behind. This in turn will degrade the efficiency of the loft - be it down/feather or artificial fibre. Liners may reduce this transfer, but not eliminate it. The only solution is to wash your bag. If you have a genuine down bag, your best bet is to get it professionally cleaned by a specialist, but its expensive - £30-40 a go. Most artificial fibre bag makers do not recommend regular washes as it damages the fibres and loft. Only Wiggy guarantees its loft for life (and honours it on bags even over 20 years old), and recommends regular washing. However, if any of you know of a similar guarantee from Snugpak, MH or any other manufacturer, I stand to be corrected!