Trying to install 14.04 as the sole OS on a Dell Inspiron 570 desktop. ISO ubuntu-14.04-desktop-amd64.iso burned to USB stick using unetbootin.

Ubuntu runs from the stick OK, and the install process appears to go OK, but when I try to boot the new installation I get the following error:

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Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/init.txt for guidance.
CPU: 2 PID: 1 Comm: swapper/0 not tainted 3.13.0-24-generic #46-Ubuntu
Hardware name: Dell Inc. Inspiron 570 /04GJJT, BIOS A00 10/19/2009
00000000 00000000 f74a3f78 c164b873 efcc332e f74a3f98 c16469ac c1826f5c
c1aa5c80 c1815698 efcc332e 00000000 c193ce80 f74a3fac c1641cec c1815b80
c1815688 fffffff8 f74a2000 c1659ab7 c1641bf0 00000000 00000000 00000000
Call Trace:
[<c164b873>] dump_stack+0x41/0x52
[<c16469ac>] panic+0x97/0x181
[<c1641cec>] kernel_init+0xfc/0x100
[<c1659ab7>] ret_from_kernel_thread+0x1b/0x28
[<c1641nf0>] ? rest_init+0x70/0x70
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The system previously ran 12.04 and other versions before that, so the hardware should be OK... (with a slighly different partitioning scheme).

There are three disks in the system:

sda is a 640Gb SATA - previously this had three partitions (/, /home, and swap). Aim is to have this with /mnt/disk2 and swap.
sdb is a 120Gb SSD - previously this had /mnt/disk2 (23Gb) and /mnt/disk3 (96Gb), not used much. Aim is to have this with / (23Gb) and /home (96Gb).
sdc is a 2Tb SATA - previously this had /mnt/disk4, aim is to preserve this - mostly media files.

When installing at the partitioning stage I've selected "Something else" and set the partitions as above, formatting the ones on sda and sdb. I've tried putting the boot sector on both sda and sdb, and changing the boot order of the disks via the BIOS. I've also tried putting a small /boot at the start of sda (shrinking the /mnt/disk2 partition). Always the same result.

Any suggestions for things to check or try very welcome - never had much occasion to play with grub, so wouldn't know how to go through the stages that the installer is presumably doing and checking each step...

I have already successfully installed 14.04 (i386, not changing boot disk) on an HP Microserver and an Acer Aspire laptop that also previously ran 12.04.

Thanks

Colin