Alternate history, Roman or Greek in Britain
I read this book in about 1999/2000 when I was in my late teens. I borrowed it from the library, and it was an older hardback (1980s most likely) I got it from the scifi/alt history section.
The cover, as far as I remember, had a couple in white toga/robes standing by a green hill.
Now, the plot.
The main thing I remember is the hero competed in arena battles using a mechanical Griffin or other mecha creature based on a mythical animal. He was in love with a slave girl, who had been sexually assaulted by a higher ranked man. I think he was lower class himself.
The book was set in Britain but it was ruled by either Roman or Greeks and the culture represented that.
I think they travelled on some sort of bike powered by Crystals, maybe using leylines as roads, but I might be confusing this with another book.
I've been looking for this book for YEARS.
Any clues are welcomed!