Rather than undertake an extensive evaluation seeking a new 'content management system' (that would inevitably require considerable customisation due to our requirements) the initial plan for the Alpha is to build on top of the GDS platform where possible and enhance this where necessary with additional (well supported) open source components. As the GDS Service Manual says we can change our minds so this isn't cast in stone for the long term but allows us to get started on the Alpha using a baseline of proven, working software.
The additional benefits of this decision include baked in interoperability between GOV.UK and the Alpha which will allow the demonstration of data sharing between the sites (especially useful due to the replacement for the Publications Hub being a GOV.UK function) and a wide pool of developers and operations staff familiar with the languages and tools at the core of the system.
The initial research and prototype provided by the Open Data Institute was based around the GDS Data API tool and some of their infrastructure choices so we have already made a small step in this direction.
GDS have provided details of their technology 'stack' at each stage of their development;