My power supply is one of these:
http://www.maplin.co.uk/Module.aspx?ModuleNo=32746
i.e. a regulated, 500mA, with a little trimpot thing to switch between voltages. It's currently set to 12 volts, and the multimeter reads that it's putting out 12.2V, which I figure is acceptable. The multimeter is quite a cheap one, but has seemed easily accurate enough in the past.
The 5V supply measures as 5.0V, so bang on. I've just re-checked the other 9V points on the board, and they all come out as 8.8V. So... Something odd with the 7809, or earlier in the chain? Here are the voltages I'm seeing from testing a bunch of random points
With the black probe at GND:
end pin of power socket: 11.5V
top left pin of switch: 11.5V
middle right pin of switch: -0.7V
positive pin of C5: 10.8V
right pin of 7809: 10.8V
left pin of 7809: 8.8V
top right pin of IC1L and IC1R: 8.8V
With the black probe on the left pin of the power socket:
end pin of power socket: 12.2V
Top left pin of switch: 12.2V
middle right pin of switch: 0.0V
Positive pin of C5: 11.5V
right pin of 7809: 11.5V
left pin of 7809: 9.6V
top right pin of IC1L and IC1R: 9.6V
Does that give any clues?