The creation and maintenance of a Register of Processing Activities (ROPA) are essential to meeting the Accountability Principle of the General Data Protection Regulation (GDPR). We evaluate a semantic model CSM-ROPA to establish the extent to which it can be used to express a regulator provided accountability tracker to facilitate GDPR/ROPA compliance.  We show that the ROPA practices of organisations are largely based on manual paper-based templates or non-interoperable systems, leading to inadequate GDPR/ROPA compliance levels. We contrast these current approaches to GDPR/ROPA compliance with best practice for regulatory compliance and identify four critical features of systems to support accountability. We conduct a case study to analyse the extent that CSM-ROPA, can be used as an interoperable, machine-readable mediation layer to express a regulator supplied ROPA accountability tracker. We demonstrate that CSM-ROPA can successfully express 92% of ROPA accountability terms. The addition of connectable vocabularies brings the expressivity to 98%. We identify three terms for addition to the CSM-ROPA to enable full expressivity.  The application of CSM-ROPA provides opportunities for demonstrable and validated GDPR compliance. This standardisation would enable the development of automation, and interoperable tools for supported accountability and the demonstration of GDPR compliance.