Complex sentence: Since 1976 (when it stood at 35 percent), the basic rate has been reduced by 15 percent but this reduction has been largely offset by increases in national insurance contributions and value added tax.
Split into simpler sentences: Since 1976 (when it stood at 35%), the basic rate has been reduced by 15 percentage points. However, this reduction has been largely offset by increases in national insurance contributions and value added tax.