Pretty normal Mik. Most cars will produce a certain amount of scale. You'd only be worried if it was a really oily sludge and your coolant level had dropped noticably.
Quite likely the scale is from years gone by. The engine may have been run for an extended period of time with no coolant/rust inhibitor which casued a build up in the coolant passages that is slowly releasing itself into the coolant. There will also always be a bit of condensation under your oil cap.