Complex sentence: The first vehicle was the Jowett Seven using an enlarged version of the pre-war flat twin first to 831 cc and then to 907 cc in 1921 when the claim to 8hp was dropped from advertising.
Split into simpler sentences: The first vehicle was the Jowett Seven using an enlarged version of the pre-war flat twin. First it was enlarged to 831 cc then to 907 cc in 1921 when the claim to 8hp was dropped from advertising.