 This study will comprise 2 phases. Phase 1 will be a dose escalation phase, designed to establish the correct dose of selumetinib. Phase 1 will be open to NF-1 participants, aged 3 to 18 years with inoperable plexiform neurofibromas PN. This phase of the study will investigate a new intermittent schedule of oral administration of selumetinib, given twice daily on 5 out of every 7 days to determine the maximum tolerated dose MTD and a recommended phase 2 dose. The starting dose level will be 25mgm2dose given twice daily. This was the MTD determined by the Paediatric Brain Tumour Consortium study of Selumetinib in children with NF1 related LGG and the National Cancer Institute study of children with NF1 inoperable PN. In both of these trials selumetinib was given twice daily, every day. The purpose of this phase 1 study will allow the investigators to define the acute and chronic toxicities and pharmacokinetics PK of Selumetinib in this population and contribute to determining the effect of Selumetinib on the growth rate of PN. 