I tend to sweat a fair bit as well Ian, I end up wearing very little on the way up, then putting on an extra layer before I hit the top, it can be a faff. The best thing I bought was running base layers, vapour-rise top and a windshirt. I carry a decent goretex jacket as well for when it really gets wet.
It almost always seems to be windy on the Beacons.
Good point there Noz, alas my budget doesn't stretch that far. On my last trip (cold summer nights-just above zero) I wore leggings and a warm base layer in side a micro fleece liner in a summer sleeping bag, I was warm enough (just) but I am a cold and fussy sleeper.
I don't have issues with tangling, but nightime toilet trips can be a nightmare with a liner. Another reason to not use a liner I guess and invest in thermal base layers.