I have just read the HMRC Digital Strategy. It's missing something important. It talks about how HMRC is becoming digital itself, but there is not much mention of its part in the greater government picture. Being one of the largest departments, I assume it should have one.
If government is genuinely to nail the lid on technology costs, it must ensure that any new service components created by departments are as re-useable as possible by the rest of government. The new government identity verification service developed by GDS is a good example of this. As departments create new digital services based on new "platform" technologies, there will be dozens, if not hundreds, of opportunities to share the underpinning components of these services.
The principle is explained in The Gubbins of Government. In the language of the video: where is HMRC's commitment to Common Gubbins? This is not something that can be left wholly to the centre (eg GDS): all departments must participate. They must collaborate to develop the most effective set of common services possible.
Open standards for interoperability are central to achieving this - but the strategy does not mention them.
It's perhaps just an emphasis thing; but whatever, HMRC should place more on contributing to the greater good. The diagram summarising their strategy at the top of this post should (if you'll forgive the Gubbins vernacular) look more like this...
