The Iron and Muscle Study

Inclusion Criteria: * Patients with established CKD (Stages 3-4) not on dialysis * Resting BP ≤ 160/95mmHg * Men and women aged 18 - 80 years * Serum ferritin level less than 100μg/L AND/OR transferrin saturation ≤20% * Haemoglobin 110 - 150 g/L Exclusion Criteria: * Pregnancy or breast feeding (Female patients of childbearing age will be asked if there is any possibility they may be pregnant. This is standard of care and no pregnancy test will be requested unless there is doubt about this. IV iron is given to many pregnant patients in standard treatment and there is no evidence of harm to mother or foetus). * Weight \< 50kg * Known allergy to iron therapy * Haemochromatosis, porphyria cutanea tarda or history of acquired iron overload * Intravenous iron within previous 6 weeks * CRP \> 50 mg/L * Serum phosphate \< 0.7 mmol/L. * Active infection * Current therapy with ESAs * Uncontrolled atrial fibrillation * Use of anticoagulants in those under consideration for muscle biopsy * Unstable angina or heart attack within the last year * Presence of solid organ cancer * Known haemoglobinopathy, myelodysplasia, or myeloma * Patients with peripheral vascular or musculoskeletal disease, who the investigator deems unable to carry out the 6MWT. * Known severe aortic stenosis and pacemaker in-situ * History of severe atopy * Severe liver disease with serum transaminases \> x3 upper limit of normal range according to local laboratory values. * Severe lung disease with FEV1 known to be \<50% predicted in last year * Known heart failure with a left ventricular ejection fraction \<45% in last year * Any other health condition considered by the local Principal Investigator in which IV iron will be contraindicated. * Insufficient understanding of the trial