Practise To Deceive by by Ann Gawthorpe and Lesley Brown

In the mid-1950s, a group of middle class women hold a committee meeting to make the final arrangements for a Christmas Bazaar. When one of them collapses and dies it seems a straightforward case of natural causes – that is
until a doctor friend, staying with Hilary the chairman, says she is the victim
of Bromate poisoning. Secrets come tumbling out as the other women try
to prove they had nothing to do with her death. A second death occurs, but
is this another case of Bromate poisoning or a straightforward heart attack?

Hilary persuades the rest of the committee to keep quiet about the Bromate
in the hope that it will not be picked up at post mortem. But the genie is out
of the bottle and events spiral out of control.