Complex sentence: Air races had been held sporadically on the Isle of Man from 1924 and the entrants were permitted to use two fields at Ronaldsway on the proviso that the entry was limited to 10 aircraft.
Split into simpler sentences: Air races had been held sporadically on the Isle of Man from 1924. Entrants were permitted to use two fields at Ronaldsway on the proviso that the entry was limited to 10 aircraft.