Right, first I should pin down what is actually being asked. They want the number of heat pumps installed relative to the number of certified engineers in the UK. I will read that as domestic heat pumps, since commercial units follow a separate certification route.

So I need two figures: how many heat pumps have been installed across the UK, and how many engineers hold a current certification, then divide one by the other.

Let me think about where those figures live. The certification bodies publish register counts, and the energy regulator publishes install volumes under the grant schemes. I could pull both and cross-check the totals against each other.

I remember the microgeneration scheme publishes a monthly dataset covering installations by technology and region. That looks like the right place to begin the count.