Sorry to drag up an old post, but:
"I've made "kidney warmers" as an extra warm layer to cover that cold meaty bit on your lower back"
Sooo, let me get this straight, you've made kidney 'warmers', which sit on the sweatiest part of a mountain biker's body and underneath a pack, out of cotton?
Er, right.
And so moving swiftly on to proper clothes for riding bikes in…
These with one of these tucked into it are my winter base, worn down to -8 degrees so far and still warm enough with the usual multiple thin layers on top. Whether it's a hard day or a bimble.
If your undershorts are cotton based, as you hint above, ditch them and try wearing ordinary padded lycra shorts (like those you'd wear under your baggies in the summer) with your existing tights. Anything with a high cotton content makes for uncomfortable chafage. No reason to ditch the baggies but most discomfort when wearing multiple shorts is caused by multiple seams – add them up and you're effectively sitting on 10mm of seam or more. Ouch.