Hello, I've noticed there a few threads like this, I hope someone can help me cos this is driving me potty!.
I read this novel at least 5 years ago and it was set in contemporary times. It was set in a coastal town in the UK, possibly Kent or Norfolk. I think the cover showed the back view of a young girl standing on a stile or groyne over water.
The main character, the mother, had a baby and was still breastfeeding. She would take the baby to the family beach hut in the evening , feed the baby to sleep then have sex with her secret lover in the beach hut. I may be remembering this wrong but I think her elder daughter disappeared or drowned whilst the mother was with her lover. I think marshes rather than the sea may have been involved.
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Sorry for the scant detail, but if anyone recognises it please let me know.