Complex sentence: The GLC also faced capping and McDonnell headed a campaign amongst Labour members to adopt this strategy in response although, unlike the local councils, the GLC faced a legal obligation to set a rate by mid-March.
Split into simpler sentences: The GLC also faced capping, and McDonnell headed a campaign amongst Labour members to adopt this strategy in response. Unlike the local councils, however, the GLC faced a legal obligation to set a rate by mid-March.