Patients will be screened for two weeks before enrollment to assess whether their ulcers are "non-healing", defined as a reduction in wound surface of < 25% after two weeks of standard best practice wound care . On enrolment and randomisation to active or placebo treatment the patient will be treated for six weeks or until 100% epithelialisation, whichever is the sooner, and then treatment will revert to standard best practice, defined as moist wound healing plus compression where appropriate. The reference ulcer will be followed up at two weeks and then at six weeks after the end of the active treatment period.