I'm trying to get a dual but set up so I can select to boot to linux
from the windows boot menu (as well as being able to select windows from the linux boot menu)

Background:
I have installed Gutsy Kubuntu on a laptop which has a single SCSI 40 GB HDD,I used to have XP + Red Hat 9.0 in a dual boot set up
and I'd used a command along the lines of

dd if=/dev/hda2 of=/mnt/floppy/linux.bin bs=512 count=1

where hda2 was instead the sda partition which had the /boot sector
of the red hat installation

the file linux.bin was put into C:\ for XP and the C:\boot.ini was modified
to default to C:\linux.bin
and included a line
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows XP" to boot to windows

I tried to repeat the exercise for (K)ubuntu Gutsy
but am reluctant to mess with the first 19.5 GB of the drive as they now
hold XP and lots of files which would be awkward to replace

after installing Kubuntu, I've got a boot menu made by the linux installation which offers me linux, linux safe mode, memtest and windows
all of which work when selected
but once in the windows menu
selecting boot to linux
calls a newly created 512 byte file (built from
the new kubuntu using the above type command) and this successfully starts
GRUB stage 1
but grub stage 2 fails to load and GRUB error 18 is reported

I would like some help understanding the fundamental problem
I've read various forum pages and they only seem to cover situations
when nothing works and the problem is either
i) old BIOS 1023 cylinders problem
ii) GRUB installation being damaged somehow etc

I believe these don't apply to me from the fact that the linux boot menu works, that grub stage 2 is in place and can be found (even though it is more than 19 GB from the start of the disk)

I know my BIOS is LBA enabled so what should I look at next?

Try grub4dos.
Else to do what you want to do, you need the Windoze loader in the MBR (not grub), and you'll need to install grub to the root partition to get the boot sector record built correctly, then redo the "dd".