Sickle cell disease (SCD) is a genetic blood condition causing long term health problems including pain and brain problems which affect quality of life. These may be made worse if patients have low night-time oxygen levels when the upper airways close repeatedly during the night (obstructive sleep apnoea). This is associated with increased pain, poorer concentration and increased kidney problems. Montelukast, widely used in the treatment of Asthma, has been shown to improve symptoms of obstructive sleep apnoea in patients without sickle cell anaemia. Investigators think this treatment could be useful in patients with sickle cell disease too. Early intervention with Montelukast could help prevent deterioration in concentration and thinking skills. The aim of this trial is to see whether young children with sickle cell disease randomised (randomise: the same as tossing a coin and not knowing whether it will come up heads or tails) to Montelukast treatment have better thinking skills compared with people randomised to placebo (tablet with no active medical ingredients - i.e. sugar pill). This means that the child could be on Montelukast treatment or he/she might be on placebo tablets.