Problem reverting to Linksys firmware

Hello all,

I have a Linksys WRT54GL V1.1 with the HSMM firmware 0.4.3 installed that I need to restore to Linksys

firmware to permit an alternate use.  I have the current Linksys version 4.3.0 bin file dated 20130308

downloaded and saved. LAN mode is NAT.

A proble arises When going to the Administration page and then the Firmware Update portion, browsing to

the saved Linksys file, and clicking Upload. The file transfers for a couple of minutes and completes by

giving the message:

Firmware CANNOT be updated
the uploaded file is not recognized

No internet connection is available to the router to use the Download feature but there is internet

available on the client Windows XP PC via wireless to another router.

What I have already tried without success:
1. The Linksys TFTP MFC application - no joy - responds "Unable to get responses from the server"
2. The Windows TFTP application - no joy - responds "timeout occured"
3. The 30/30/30 reset
4. The NIC in 10MHz mode

Your suggestions would be much appreciated.
73,

Richard