I'm a big fan of the original UCO lantern - it's nice to have a warm light (and a bit of heat) in the winter. Due to the tunnel design of my tent the weight of it isn't an issue for hanging it, as the ceiling hanger points are all directly underneath a pole attachment point. I generally hang it at the foot end of the tent mainly so I don't bash my head on it when messing about near the door.
Regarding the original question about sleeping bag warmth, Oliver have you ever used your PD600 in cold weather yet? I have a PD400 which I was comfortable in wearing just a base layer (a helly hanson warm long sleeved top and some 'longjohns' type things) and, yes, a balaclava in temperatures cold enough to freeze the dew onto the outside of the tent. Everyone is different, which is why I was wondering if you've had any experience with it. One thing which does tend to make the pipedream bags quite warm for their weight is thier snug fit - less air to heat and fewer draughts.
Other than that, I think everyone has been been over most of it. I'd say the main point is that you can have the fanciest sleeping bag in the world but you'll still feel cold if you're losing heat into the ground, so using a standard el-cheapo closed cell foam mat under your normal one would be a very good idea.
I started off using a convertube but got a widepac in the end because my rucksack has the option of hanging the bladder in the gap between the rucksack and the frame, which is a more efficient use of the available space. I have the 3 litre version of the widepac (although I would never fill it that much unless using it for water storage while I'm camped) and haven't had a problem with it.
Having decided that I wanted a Mountain Hardwear Lamina 35, I was searching for deals and discovered that Cotswold have a huge discount going on for the Lamina 35 and Lamina 20 in their sale. The Lamina 35 is £60 which is at least £15 less than I can find it anywhere else, and it's free delivery. And if you go via Quidco you get 6% cashback which effectively makes it £56.40 - pretty good!
Base layers shouldn't make sweating ineffective either. Sweating cools you down by energy transfer - your sweat is heated to your body temperature, so when it is removed (regardless of how it is removed) that energy its lost and you cool down. Base layers remove sweat so, as long as they can then breathe themselves so that the moisture can be released, you will still lose that energy. You can tell if this isn't happening if your base layer ends up soaking wet!
You might find that you're not cooling down enough if you're wearing a warm base layer in a warm sleeping bag, but that's a different problem entirely - over insulation. It's solved by ditching the base layer and/or opening the zip!