Complex sentence: Although very hostile to Thomas, Earl of Lancaster, Bartholomew helped to make peace between the king and the earl in 1318, and was a member of the middle party which detested alike Edward's minions, like the Despensers, and his violent enemies like Lancaster.
Split into simpler sentences: Although very hostile to Thomas, Earl of Lancaster, Bartholomew helped to make peace between the king and the earl in 1318. Bartholomew and his associates formed a loose grouping which has been referred to by modern historians as the "Middle Party", who detested alike Edward's minions, like the Despensers, and his violent enemies like Lancaster.