Source of Volo non valeo as quotation?

Volo Non Valeo is inscribed in the family crest over Castle Howard in Yorkshire. According to historians there, it was the family motto assigned by King Charles II, because when he asked the Duke why he hadn't been faithful to Charles I, who was beheaded in the Cromwell era, the Duke told Charles II: I was willing but unable. Charles II told him he would allow him to retain the title and castle, but forever after would have to adopt that motto: Volo Non Valeo.