Solved!!
There was corrosion on the connectors inside the output speaker jack socket that meant when there was no jack present, the signal to the speakers was cut. Scraping the oxide off allowed restoration of signal. It only took me five and a half hours!

Hi. I just got this amp off eBay. It was sold as faulty. Powers up and all lights shine and flas etc. but no sound out of speakers.

Speakers fine.

The extension speaker output can be used to put into another amp. This demonstrates that all effects and preamp are working fine on all 3 channels. The output from here is just too low to drive speakers without further amplification.

Strangely - as I was removing the jack plug from the other amp's input -the sprakers in the crate came on! If I connect the hot wire (tip) from the external speaker output to the ground of the crate amp - the internal speakers work nice and loud, but there is also a lot of noise (to be understood when connecting a ground to the power amp section i presume)

So here lies the problem. What can I do to restore the signal to the power amp section without adding all that extra noise?

TIA
D.