Grub doesn't find Window7

[R Kimber]

I'm running a dual boot PC with Windows7

I've been forced to reinstall my 15.04, and did so from the live disk
(previously it had been upgraded via apt from 14.10.

However, the grub menu no longer includes Windows7.  In the re-installation
I (perhaps mistakenly) put grub on the main Ubuntu disk (sdb) rather than
the Windows disk (sda). So I tried to re-install grub using

sudo grub-install /dev/sda

But this gives:-

Installing for i386-pc platform.
grub-install: warning: this GPT partition label contains no BIOS Boot
Partition; embedding won't be possible. grub-install: error: embedding is
not possible, but this is required for cross-disk install.

Which I don't understand. Why i386-pc?

I have tried the "boot-repair" disk, but it won't work - it complains about
the PC being booted in legacy mode and I need to boot it in EFI mode.
However, the bios (Gigabyte UEFI DualBios, latest version) doesn't seem to
have a way of forcing the PC to boot in EFI mode - at least I cannot see it.

I am able to boot windows if I select the disk with the Windows Boot
Manager directly from the bios, by over-riding the normal boot sequence, so
both Ubuntu and Windows installations are working OK and their respective
boot arrangements seem to be OK.  I just need to get grub to boot windows
from its menu.

I'd be very grateful for suggestions as to how to proceeed.

- Richard.
-- 
Richard Kimber