Is there a legal requirement to comply with or be certified to ISO 27001?

There is, generally, no direct legal requirement as such.  Organisations choose whether or not to implement the requirements of ISO 27001 based upon the benefits that would be gained by doing so.  

However, you should pay close attention to any contractual obligations you may have for protecting the information of clients and other stakeholders.  

There is an increasing trend where customers require third party suppliers to implement or certify to ISO 27001, thus making it a legal requirement, by way of a contract.