I had a look at uboot as it is a closer version number to the firmware I have.
The readme says:
IT IS NOT APPLICABLE FOR ANY OTHER FIRMWARE VERSION AS THE KERNEL IS ASSUMED TO BE 1.04 COMPATIBLE
This concerned me that I might end up with a bricked device.
Do you reckon porkupan's will be OK?
Incidentally the reason I went for 10.5 rather than 10.4 is that Sony seem to have removed the updater that goes to 10.4.
If anyone has this software lying around it would be useful if it could be uploaded somewhere and linked to from the wiki. The current link goes to the (dead) sony updater link.