Cold knees causing problems

I'm attempting to keep cycling through the winter months and as the temperature drops my knees are suffering and consequently more prone to aches, pains and possible damage. I invested in a pair of Endura Endura Thermolite Pro padded Bib tights, but despite the double layer of fleece at the knee they're not to good going into single figure temperatures. Subsequently I bought some Endura Stealth Extreme Tights which claim to be waterproof / windproof and have a lovely fleece lining and should be good going into below zero temps. However having ridden in them and although they appear better than the bibs my knees still feel the cold at around 4 - 6 degrees C so I suspect problems will arise if it gets icy. Does anyone have any good advice about how to keep the knees warm in tights, or any winter knee care wisdom generally? Advice on what might be the best winter tights would be welcome too if anyone has any?