Layering (lower half) for long chilly rides I was riding all day on Sunday in temperatures that peaked a few degrees above freezing, but dry apart from the odd snow or hail shower.

It was cold enough to need layers. As I tried not to hang around outdoors, my top half was fine with just a short-sleeve jersey under bib tights and a long-sleeve jersey over, plus a buff round my neck, double gloves and home made wrist/forearm warmers. My legs and feet were fine too - 2 thin pairs of socks and pedalling effort. I didn't even need my shoe covers on.

But in between was an issue. I didn't want anything under the good chamois of the bib tights, I didn't want two layers of gel padding, but I wanted more insulation than just the bib tights. So I went for foam-padded shorts (the foam compresses quite quickly) over the tights. From an insulation point of view they were ideal - when they were in place, which was rarely. Every time I got going they caught on the saddle, and even just pedalling along seated they crept down. I think the problem is partly that cheap shorts don't grip, and expensive ones are normally gel. Loose-fitting stuff tends to get caught up and form lumps that rub, and it's nice to avoid the drag. 

What are some tried and tested ways of layering for all-day rides in these temperatures? We've got a few questions on layers for cold commuting, but 12 hours door-to-door/200km is a bit different, and I expect the solutions will be, partly because of the need for something that works against the saddle. I regard minimising sweating as important; even decent wicking clothing gets cold fast if you're in the wind and damp with sweat.
For the kind of riding you describe, the best choice are windproof tights. You should look for the kind that has windproof membrane in the front, breathable fabric in the back and most importantly no padding. Underneath them you wear shorts or bib tights with chamois. 

These are available from many cycling-specific brands, but the ones intended for nordic skiing work too.

If the membrane gets too warm, the next thing are winter running tights made of brushed lycra. Running-specific because they come without padding.