It is these words from the coroner which I believe have led to the HSE prosecution. I could be wrong but he was pretty clear in his assertion...
"Martin-Baker knew the parachute mechanism could jam if the nut and bolt were too tight as early as January 1990, the inquest was told.
The manufacturer warned some air forces but did not warn the MoD."
This comes from the BBC report of the coroner's inquest. Incidentally not too glowing praise for the RAF either...
http://www.bbc.co.uk/news/uk-england-lincolnshire-25943211