Gallowglass, paperback/barbara vine

When Sandor snatched little Joe from the path of a London Tube train, he was quick to make clear the terms of the rescue.

I saved your life, he told the homeless youngster, so your life belongs to me now.

Sandor began to tell him a fairy-tale: an ageing prince, a kidnapped princess chained by one ankle, a missed rendezvous.