SOLVED. Teen book, boy reluctantly chosen to be an oracle, 80s/90s, probably British [s]

[Trillian]

A teen book about a boy (I think) who was chosen to be some kind of priest or oracle. I think he was working or training at a temple and was surprised to be chosen. He didn't want to be the oracle and didn't believe in the rituals. It had a kind of fantasy/classical or alternate world setting.

The one thing I remember clearly is him being told to eat 'the fruit of mortification' and replying 'it's a sodding onion'.

I read this some time in the 90s, and it's probably British.

[Anna]

This is Divide and Rule by Jan Mark (who was, indeed, British). Fantastic book - wonderfully detailed writing and characterization.

[Trillian]

Wow, that was fast, thank you!