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?
Paul@slowe I have not been able to find one after a lot of searching, and I think it would be difficult anyway because you would need a set of API keys for every user, which you can only get by going through the HMRC testing and sign off process, and accepting liability for penalties if anything goes wrong