Complex sentence: The Squadron were based at Debden in Essex and commenced training using its temporary mount the Gloster Gladiator (the RAF's last Biplane fighter) however on the 4th of September the first Hawker Hurricanes began arriving in numbers.
Split into simpler sentences: The Squadron were based at RAF Debden in Essex and commenced training using the Gloster Gladiator (the RAF's last biplane fighter). On 4 September the first Hawker Hurricanes began arriving in numbers.