Whoa there, just a minute
"Digital marketplace is reaching a stage which allows us to consider making it the route for G-Cloud purchases,"
It may be true that the digital marketplace is ready, but I seriously doubt the government is. You need to hold your horses while you still have some left (what with all the ones that have already bolted long ago) and give more thought to the whole scheme.
And I don't care how much thought has been given, since past projects clearly demonstrate that enough thought has never been given to any government project.
So get back to the drawing board and make sure that all contingencies have been planned for. Then plan for the ones that haven't, for a change.