Complex sentence: Alabaster carvers were at work in London, York, Burton-on-Trent but the largest concentration was in Nottingham and this has led to all the English medieval output being referred to as "Nottingham alabaster".
Split into simpler sentences: Alabaster carvers were at work in London, York and Burton-on-Trent, but the largest concentration was in Nottingham. This has led to all the English medieval output being referred to as "Nottingham alabaster".