For the quicie answer:  Lance did it.

He met Gladys at here job at the resort and made up to her.  He was using
a false name and she fell in love with him.  He inveigled her into putting
the taxine into Fortescues marmalade thinking it was something else for
quite another purpose.  His main purpose was to get his hands on the
Blackbird Mine, where a uranium deposit had been found.  The blackbirds
from earlier were put there by Percival's wife who was the daughter of
the man who was Fortescues partner in the Blackbird.  He died under
mysterious circumstances, and his wife raised her daughter to take
revenge on Fortescue.  But the daughter found a better revenge than
killing him.