Laurence Moody – What Became of Jack and Jill?

Laurence Moody – What Became of Jack and Jill? (Sphere, 1971: originally Robert Hale, 1969, as The Ruthless Ones)

Blurb:

Jack loved Jill. And Jill loved Jack – especially the money he would inherit when his grandmother died. But the old lady kept an annoyingly strong hold on life – and Jack’s purse strings. So Jack and Jill reached a decision: grandmother had to die. And with all the cunning of two teenagers with no morals they set about frightening Gran to death.