How to develop software for MTD, CT600, filing etc

Hello we are looking at creating a platform for small companies to allow them to manage their accounts etc, and a key part of this would be the ability to submit VAT (MTD) returns, filt CT600 + iXBRL etc. How do we register as a software developer? How can we access specifications / protocols and get access to test resources? Finally how do we get certified so we can get listed on the HMRC pages? Thanks, Jim

Hi Jim,
Please see:-
HMRC third party tax software and application programming interface (API) strategy 9. Contact us 
Thank you.