When you attempt to power on your Humax the standby orange light goes blue for a moment, and then powers off automatically again.
The reason for this could be a bad boot sector on the hard disk.
To fix this, remove the hard disk cable that connects to the mother board, power the machine on, then whilst it is showing your channel on the display, connect the hard disk cable back onto the motherboard. Then ftp to the machine using humaxftp / 0000 as the default username and password, and change directory (cd) to "my videos" and then list the contents of the directory using a dir command. After doing this you should be able to put the machine back into standby, and wait until you hear the hard disk spin down and stop, and then switch off at the wall and the back on again.
Note: There also appears to be a telnet command line interface to run disk repairs on the Humax but I wasn't able to get this working myself, and in the end did not need to do this.