Gigabyte have told me that the BIOS chip is made by SST, and the model name is SST25LF040A BIOS ROM which is PLCC32 type.
The data sheet and details are at
http://www.sst.com/products.xhtml/serial_flash/25/SST25LF040A
I am now very confused because I have PLCC32 sockets and these are nothing like the BIOS chip which has 8 connections.
(I have two PLCC32 sockets and four unused flash chips to give away, if they are no use to this version of the motherboard).
I will probably try to source some of these chips, just in case the rest of the problems can be solved.
Chris Lingard