A 65-year-old man with a history of ischaemic heart disease and type 2 diabetes is prescribed atorvastatin 20mg ON. After 3 months, his lipid profile is: Total cholesterol 4.8 mmol/L, LDL-cholesterol 2.8 mmol/L, HDL-cholesterol 1.1 mmol/L, Triglycerides 2.0 mmol/L. According to NICE CG181 (Cardiovascular disease: risk assessment and reduction, including lipid modification), what is the target LDL-cholesterol level for secondary prevention?