+1 best sleeping bags i've used (and that's a fair few)
I have the Minimus which is rated to 5c, at 5c i couldn't say i was toasty hot, but i certainly wasn't cold in any way.
For 4 season use i'd recommend the
Minim 400 585g and rated to -5c, throw on a down jacket, some warm socks and trousers and i'd happily spend all but the very coldest UK winters in it.
If you are a cold sleeper though the
Hispar 400 would be my next choice, at 655g it's still crazy light especially considering it's rated to -9c comfort use.
Again i'd be happy to use that through any night outdoors through a UK winter.
If you want something warmer still take a gander at
their range, you can also
design your own sleeping bag
Even if you don't want to design your own you can still spec various outers for the standard PHD bags, although i wouldn't trust the waterproof outers enough for bivving they are waterproof enough to offer a bit more security for accidental spills and/or condensation dripping off tents.
You can also spec where you want the zip and how long you want it, i always prefer a full zip on warmer bags as i just can't sleep if my feet are hot, so i unzip the bottom and pop my feet out.
One other thing, not sure on your experience so sorry if i'm teaching a chicken to lay eggs, but a good quality sleeping mat is just as important as a sleeping bag.
The 2 i would recommend in no particular order are the
ThermARest NeoAir XTherm or the
Exped DownMat 7
The ThermARest is nearly half the weight of the downmat and they're both cracking mats, choice is really down to the individual, some prefer the Eped mats and find them more comfortable, others the Neoairs.
For me personally i prefer the Exped mats as the 2 outer tubes are slightly large in diameter than the inner ones, so it tends to help stop me rolling off the mat in the night.
As i say though other prefer the Neoair it really depends on how you sleep.