GMB members have been taking a campaign of industrial action since April, taking in a dozen days of strike action.
The members are employed by ISS and work as caterers and cleaners across the four hospital sites operated by South London & Maudsley NHS Trust.
The dispute centres around pay, terms and conditions, as these members have had pay rises withheld for years, as well as facing site closures, increasing workloads and bullying managers.
The union are expecting hundreds of members to attend – a similar demonstration at ISS headquarters in Canary Wharf earlier in the campaign saw 200 members assembling together.
