Quite easy to make, and it cost maybe £10 - and that gives me enough bits to make a second spare circuit board - the weather proof housing was the most expensive bit at about £4 - and a third cct board to 'play around' with UV.
The current graph can be found here, click on the solar button. (also on the 'gauges' and 'now' pages)
Note that the theoretical value has stepped down this afternoon, I did have the transmission factor set at 0.85 as part of the tinkering, I reset it today back to the default 0.8.
There are lots of white clouds around today, so in theory the sensor should be over-reading, but it seems to be hovering around the max in full Sun, so maybe it will need tweaking up a bit.
Edit: Actually the value seemed to jump up as the Sun went in and out of clouds. Trees started shading the sensor from direct Sun around 16:30
![Sad :(](./images/smilies/icon_e_sad.gif)