Parkinson's disease (PD) is the second commonest neurodegenerative disorder, affecting over 145,000 people in the UK. Initially, PD patients experience slowness of movements, limb stiffness, and tremor. With progressive loss of neurons over time, many patients start to experience balance and walking problems, and falls, which are resistant to currently available treatments. Falls can lead to fractures and nursing home admission, and can significantly shorten patients' life expectancy. In this pilot study, the investigators will investigate the effects of spinal cord stimulation (SCS) on gait and balance in PD. Some open-labelled studies have shown possible beneficial effects of SCS in PD, although it is uncertain which type of PD patients will benefit most and which stimulation parameters work best. The investigators will assess the effects of SCS on posture and gait using a series of clinical, laboratory, imaging, and wearable measurements. The participants will receive a percutaneous implantation of a spinal cord stimulator to minimise the possible adverse effects related to the surgery. The SCS will start one month after surgery. The investigators will use a double-blind cross-over design. The participants will receive three different stimulation parameters, including sham stimulation, in a randomised order. The participants and the assessors will be blinded to the stimulation parameters.