Thanks Cobbler. My main problem was not being sure if it was a power supply issue - or some DSP hang-up. It seemed like there wasn't the power getting to the unit to boot it up, but as with all the complications of a computer boot-up, I don't have the knowledge to tell. There doesn't seem to be a way of re-booting the unit, although I did read in another discussion (about a unit hanging up during software upgrade) that you can do this by pressing Recall whilst powering up? Is this the case - and does it affect patches, settings etc?
Also, I don't know if there's a Fractal Audio guarantee requirement not to have opened up the casing - and still don't, although the instructions from G66 require me to open it up, so I guess it will be OK.
The effectiveness of Support plus this Forum with the opinions, helpfulness and expertise of people here are a very big reasons why me for one decided to make the change to a DSP. I'll post how all this goes, in case anyone else has a similar problem.