Scrutable

The National Radiological Protection Board.

It's gone. Merged apparently with this lot: https://www.emfs.info/. I see nothing on their website about ionising radiation just electro magnetic fields.

The NRPB were responsible for identifying the risk from radon in older British housing stock, they served a useful purpose, a purpose that seems to have disappeared.

Who would I refer a radiological (ionising radiation) hazard to?

The Environment Agency or UKAEA, depending on suspected source.

I thought NRPB were merged into HPA, which became PHE, which became NIHP.

There's this lot, who claim to be PHE but seem to be the commercial for-profit wing of it.

For radon specifically, try https://www.ukradon.org/ which also seems to be from PHE.