Complex sentence: The highest standard 0900 GMT-0900 GMT rain day record however remains 279 mm at Martinstown, Dorset set on 18 July 1955, as much of the historical data is recorded in this way.
Split into simpler sentences: The highest standard 0900 GMT-0900 GMT rain day record however remains 279 mm at Martinstown, Dorset set on 18 July 1955. Much of the historical data is recorded in this way.