Thanks for the reply, -the Utility programme reported that the downloaded .hex file was for a 100 series, when it was a 300 series file. I continues with the install anyway and it seemed OK.
I'll try to give that a look later. Which hex file did you use?
What do you mean "flashed" more than once? -Sorry I'm new to this! Should I uninstall and try again?
The bootloader installation on the H300 requires flashing the firmware file. That is, the hex file has to get modified (which is done by Rockbox Utility), placed on the player with the correct file name (also done by Rockbox Utility) and afterwards flashed to the players builtin flash memory the player is booting from. The player does not boot from disk, so the original firmware can work even on a player without a disk (it won't find any music to play, though, so doing so is somewhat useless). Rockbox needs to follow this procedure, which means that at least the bootloader has to run from that flash memory. For easy upgrading (and as it's the usual way on all other players) only the bootloader runs from that flash memory, everything else is loaded from disk. This is a major difference to the original firmware, though it makes no real difference for using the player, it's just a technical difference.
Flashing has to be done using the firmware upgrade functionality of the original firmware. Rockbox Utility should tell you about that step. However, there are cases known where this flashing procedure had to be done twice before it worked.