Complex sentence: It seems that Clarence did not realise how big the Franco-Scottish army and decided rely on the element of surprise by attacking immediately, he discounted the advice of his lieutenants Huntingdon and Gilbert Umfraville to consolidate his own force and position.
Split into simpler sentences: It seems that Clarence did not realise how big the Franco-Scottish army was as he decided to rely on the element of surprise and attack immediately. He discounted the advice of his lieutenants Huntingdon and Gilbert Umfraville to consolidate his own force and position.