I read a book in the early 1980s (I think, though maybe a bit later) which was set in the Stone Age (approximately) in Southern England.
At one point the hero is enslaved and put to work in a mine named "The Trundle" under the South Downs (chalk hills along the south east coast of England), iirc.
Later, he wreak revenge on someone by laying a trap baited with a deadly biological toxin (maybe ergot) which the villain obligingly falls for, ingests and dies a suitably gruesome death. I think there was some blurb on the back that mentioned that it contains Britain's earliest biological attack.
I imagine the author was British. I think the paperback (which was larger format than Penguins, etc.) was published by one of the publishers who specialised at the time in modern authors, e.g. Picador.
Does anyone recognise this story?