Thank you to both Silyboy and Cream for the great information supplied on this post.
I have discovered that my radio is the BM53 variant, but looking at the pictures of the PCB, appears to be identical to the BM54 with a single tuner front end instead of the twin tuner of the BM54. I have not been able to locate a firmware update for the BM53.
My reasoning at this time is that the C16x and 29f400 are part of the ibus MCU control, separate from the Tuner functionality, and as such the 29f400 firmware load for the BM53 should be the same as for the BM54.
I'm going to try to flash my BM53 with the BM54 flash load using the bootstrap loader method on the assumption that the firmware will be identical.

DOES ANYONE ON THIS BB KNOW FOR SURE IF THE FIRMWARE FOR BM53 AND BM54 ARE THE SAME, OR AM I ABOUT TO MAKE A TERRIBLE MISTAKE?

Alternatively, if BM53 is different firmware to BM54 and anyone has access to the BM53 firmware update, could they possibly PM me.

Any advice would be gratefully received.

thanks to all.