The Night in Question by Susan Fletcher

As I said to Susan, 'I don't read cosy crime, but this is not a cosy crime book'. This is an adventure with one of the funniest, bravest, most marvellous female protagonists I've come across in a long time. Florrie Butterfield will melt all over your crumpet of a heart.

From the back

Florence Butterfield has lived an extraordinary life full of travel, passion and adventure. But, at eighty-seven, she suspects there are no more surprises to come her way.

Then, one midsummer's night, something terrible happens - so strange and unexpected that Florrie is suspicious. Was this really an accident, or is she living alongside a would-be murderer?

The only clue is a magenta envelope, discarded earlier that day.

And Florrie - cheerfully independent but often overlooked - is the only person determined to uncover the truth.

As she does, Florrie finds herself looking back on her own life . . . and a long-buried secret, traced in faded scars across her knuckles, becomes ever harder to ignore.