OK Andy, I'm interested, please take this as 'asking'. (Sorry Iain if this is hijacking your thread)
We have a family legend about descent from Robert the Bruce, on the wrong side of the blanket of course. It's totally unproveable but I've always believed that in a small corner of Scotland where families were based, and intermarrying, for generations, and with him acknowledging so many illegitimate children, probably over the centuries everyone has some sort of link to him, however tenuous.
Would love to learn some factual background for my theory!