Obtaining hourly weather data, including direct and diffuse insolation I really need some reliable measured hourly weather data for a few sites across the world. It needs to include solar diffuse and direct values or their equivalents.

I've tried the usual sources: Met Office, Weather Underground and Weather Analytics but none of them provide what I need. They are either not hourly, don't contain solar radiation, or have difficult licence/ access restrictions. This is for some research to determine if there's any relation between position of sun, temperature, relative humidity, wind speed and direction. So I'd need a couple of years data for a couple of locations. Aggregate data isn't much use I'm afraid.
If reanalysis data (ERA, ECMWF, CSFRR) is too aggregate for you, try looking for the datasets used to calibrate photovoltaics models (PVGIS, PVWATTS and the like) - they include both direct and diffuse insolation.

Also, follow the citation trail for papers that describe the calibration of the PV models, and the reanalysis models - some of the datasets you are seeking will be somewhere in that trail.

Barry Carter has pointed you at the NOAA ISD-Lite dataset, and I'd like to extend that pointer to the whole Integrated Surface Data from NOAA, which is enormous: 20,000 global stations. Because it's an amalgamation of many different sources, they don't all share the same metadata, but this pdf lists the breadth of the metadata: the insolation variables are on pages 64-77.