 When Dose Group 4 has completed the seven day Dosing Period, and subject to assessment of clinical safety and pharmacokinetics, patients will then be invited to continue into Part B, which is the double blind, randomized, placebo-controlled phase to investigate the efficacy and safety of GWP42003-P. There will be a washout period of at least two weeks between the last dosing day in Part A and randomization in Part B. 