This book I read in the 1980’s at my school library  (small primary school, under 30 kids) in a very tiny town in country NSW Australia.

All I remember is that it involved a girl and boy set in England?. I think he was infatuated with her.. either he kidnapped or she went with him willingly and he took her to the moors and there was a cave there. He made her do a blood pact to tie them together. I am pretty sure he had dark hair. Thats all I remember!