Now that 2old mentions it, I am pretty certain that his post was the source of my information.
I tried this with an Amstrad DRX100, but I couldn't get the JTAG interface to divulge anything except the chip ID. I think the available JTAG firmware reader programs rely on a using the DCU (diagnostics control unit) that is built into more recent STi5xxx processors. The ST20-TP2 in my Amstrad doesn't have this so maybe this is why it didn't work. The JTAG trick works fine with a cheap FTA receiver that I have with the STi5505 processor.
I was going to try an OS-link with the ST20-TP2 but I couldn't get my OS-link interface to work. Maybe another day when I have more time, I will have another go at trying to sort this out.
By the way, some other were people using PC cards to download the software updates that get sent out to Digiboxes. This seems a painless way of getting the firmware, as you don't even need to have the Digibox!