The HSE Say:
31. It is not appropriate for personnel to stand on the guardrails. In addition, it is advisable not to
lean out of the platform when it is raised and, so far as is reasonably practicable, the platform shall
be positioned to prevent the need for people to lean out when carrying out their work. If people have
to lean out of the platform to carry out work at height then they shall wear harness and lanyards that
when linked to the platform’s harness anchorage points prevent them from overbalancing and falling
over the rails.
Full Guidance here:http://www.hse.gov.uk/workplacetransport/pm28.pdf
Hope this helps.