Some time in the early 2000s, I borrowed a children's book from a UK library. It was written in English and set in the south of England, so probably by a British author. I don't know how recent it was, but probably not from any earlier than the 1990s.
The main character is a boy who's going with his family (mum, dad, and younger sister) from their home in the Midlands to the east coast town of Skegness, probably to visit some relative. I don't remember much about the plot, but witches play a main role. I thought the title might have been something like "The Witches of Skegness", but searching for this only brings me to a real-life witches' shop in Skegness. Pirates also appear in the story: I remember a scene where a crew of pirates board a bus, with hilariously anachronistic consequences, and another scene where one of the pirates says "Avast!" and one of the modern guys responds "A vast what? Oh, you mean stop."
Not relevant to the plot at all, but another thing that's stuck in my mind: during the car ride to Skegness, I suppose the adults are talking about diseases, and there's a couple of funny exchanges between the father and the younger sister: "Dad, what's rabies?" "Like babies, but not as nice." [...] "Dad, what's leeches?" "Like peaches, but not as sweet."
I think salt also plays an important role. Maybe there's some special salt with magical powers for the witches or the pirates?
There's a climax where everything's about to go wrong but suddenly, by accident, the boy's mother (who's been completely unaware of all the goings-on with witches and pirates) gets a load of magical powers and somehow saves the day. At the end of the story, when the family has gone back to their everyday life and she's seemingly lost the magical powers she briefly gained, there's another funny exchange where she seems able to compel or hypnotise her husband into doing something (mowing the lawn?), suggesting that some of those powers remained after all.
What was this book?