Just to say, the solution was to remake the initrd manually, which has been suggested. The magic combination was this:
mkinitrd /boot/initrd-2.4.21-27.0.4.ELsmp.img 2.4.21-27.0.4.ELsmp --with=scsi_mod --with=sd_mod --with=libata --with=sata_sil --with=3w-xxxx --with=ata_piix --with=sata_sis
(remembering to remove the RPM installed initrd image file first)
Not entirely sure why this particular RPM kernel installation didn't include the driver, but it seems to be okay now.
Thanks to all for their suggestions and advice.
Regards,
Martyn