Was the film 'The Englishman Who Went Up a Hill But Came Down a Mountain' based on a real event? If not, has anyone ever gone to great lengths to make a mountain bigger?

Apparently it is a true story. It took quite a long time to find a filming location that could pass for Taff's Well in 1910; the original location, to the north of Cardiff, now has several large wind turbines and electricity pylons prominently placed, and the village is surrounded by a modern industrial estate.

https://www.imdb.com/title/tt0112966/trivia

Cringing Cult of Celebrity (Famous residents and ex-residents.)
David Jason lived here for a while. Of course the film "An Englishman Who Went Up A Hill And Came Down A Mountain" was based on the Garth Mountain and the author came from Taffs Well. A smattering of media types also live in the environs.
http://www.knowhere.co.uk/4837_heroes.html


Monger, in the epilogue to the book, says that about five years after the event there was a new edition of the map, which showed "Ffynnon Garw Mountain - 1002 feet." Soon all the villagers had a copy in their homes. It is a good thing they did not see the 1921 edition (which is in the library). It says "Garth Hill - elevation 1000 feet." Although that is the 1921 edition, the small print says the leveling was revised in 1899 and partly revised in 1915.
http://www.surveyhistory.org/englishman_who_went_up_a_hill.htm