I ended up finding a transformer from an old radio... It was actually centre tapped, using the taps in parallel with a diode on each, in the radio.
So, taps in series gave me about 13.6volts AC unloaded, resulting in 18volts DC once rectified, that dropped to 14volts with the load of the signal generator.
Ah well, close enough and it didn't cost anything.
However, this only seems to do 2volts rms output max... the 100mv range doesn't work, and the 10/1mv ranges both seem to output the same voltage.
Square wave also isn't 50% duty cycle, its fooked a bit...
BUT!
As I mentioned, I only wanted this for testing audio, so squarewave doesn't matter, and I really only ever needed the 2.5volt range anyway.
So, she'll be right!