Stuart 
I asked HMRC what open source software there was for "Making Tax Digital". Their answer: use their software-finding tool. I'd asked them the question because their tool doesn't make that information available.
Does such a thing exist?
Doug Belshaw@slowe According to my Little Robot Friend, this is the current list.
None are Open Source, primarily because of how HMRC's API system deals with credential management. To interact with the Making Tax Digital production API, a software application must obtain production credentialsvia the HMRC Developer Hub.
These credentials are, by HMRC's policy, meant to be kept secret and are not intended to be shared publicly - which is a bit of a problem for an OSS project.
Right now, AFAIK, each person using OSS would have to obtain their own credentials...
Derick Rethans