What was the change in the net pension plan surplus on the balance sheet?
{"": {"2019": "\u00a3 million", "2018": "\u00a3 million"}, "Schemes in net asset position -- UK defined benefit pension plan  Staff Plan": {"2019": 7.8, "2018": 1.1}, "Schemes in net asset position -- UK defined benefit pension plan  Cash Plan": {"2019": 1.0, "2018": 0.9}, "Schemes in net asset position -- ": {"2019": 8.8, "2018": 2.0}, "Schemes in net liability position -- UK unfunded plan": {"2019": -0.5, "2018": -0.5}, "Schemes in net liability position -- Net pension plan surplus on the balance sheet": {"2019": 8.3, "2018": 1.5}}
net_pension_plan_surplus_2019 = df["Schemes in net liability position -- Net pension plan surplus on the balance sheet"]["2019"]
net_pension_plan_surplus_2018 = df["Schemes in net liability position -- Net pension plan surplus on the balance sheet"]["2018"]
answer = net_pension_plan_surplus_2019 - net_pension_plan_surplus_2018