> That should be so obvious that I wonder if it was DIY by the pilot.

I don't know how the regulatory environment is in the UK for experimental craft (this is considered to be "experimental" category in the US and Canada), but yes, the idea behind an experimental is that everything is DIY.

I have an experimental, and I can do close to anything I want. What I can't do is complain when my plane crashes because I installed a part that isn't fit for duty. I, as the owner and operator, am the one that signs off on the airworthiness of the plane.

E.G. If I install a Cessna part on my plane, and that is the cause of a crash, that is my fault from the point of view of the FAA.

There may be legal considerations outside of airworthiness and flight rules, but as far as the FAA is concerned (or would be if this had happened in the US), the manufacturer of a part is off the hook once the thing is installed on an experimental.