Cannot get Grub2 to recognise Vista

Try first to reinstall Grub2 from CD:
https://help.ubuntu.com/community/Grub2#Reinstalling%20from%20LiveCD

Done. New versions of various files have duly appeared. Vista is still sitting in its partition, but it does not appear in the boot list. How do I get it there? The boot/grub files mutter about entering something in boot/grub.cfg via etc/grub.d/40_custom, but it is not clear to me what I should put there, nor how into a read-only file. Or is there some better way of doing it?
  Should I reinstall Vista into that partition?

Do not reinstall Vista. That will kill your Grub and replace it.

Back to that link above and you can see how to add your Vista boot back in.
https://help.ubuntu.com/community/Grub2#Creating%20the%20Custom%20Menu

There is also a fairly detailed walkthrough here:
http://linux.koolsolutions.com/2008/12/28/windows-xpvista-dual-boot-does-not-boot-from-grub2-or-grub-pc/

HTH's.

Run command: sudo apt-get install os-prober ; os-prober
and check if Vista is detected.
If it fails, download latest version: http://packages.debian.org/unstable/utils/os-prober
and retry.
If it fails, we will have to edit manually file etc/grub.d/40_custom

This latter produced:

dave@dave-Dimension-E521:~$ sudo apt-get install os-prober
[sudo] password for dave:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  localechooser-data
Use 'apt-get autoremove' to remove them.
The following NEW packages will be installed
  os-prober
0 upgraded, 1 newly installed, 0 to remove and 338 not upgraded.Need to get 23.0kB of archives.
After this operation, 184kB of additional disk space will be used.
Get:1 http://gb.archive.ubuntu.com/ubuntu/ maverick/main os-prober i386 1.39 [23.0kB]
Fetched 23.0kB in 0s (122kB/s)
Selecting previously deselected package os-prober.
(Reading database ... 175315 files and directories currently installed.)
Unpacking os-prober (from .../os-prober_1.39_i386.deb) ...
Setting up os-prober (1.39) ...
  then:
dave@dave-Dimension-E521:~$ sudo os-prober
/dev/sda1:Ubuntu 10.10 (10.10):Ubuntu:linux

var/lib/os-prober/labels contains “Ubuntu 1“, and was modified when I reinstalled Grub2. Today’s exercise seems not to have changed it.
  os-prober’s remark is about sda1: Ubuntu has an entry in there, but could various changes in the partitioning have removed whatever Vista had there?

Could you run following procedure, so I will collect data enabling me to propose an entry in 40_custom
-----------------
There is a standard script provided to analyse boot configuration.
 Could you download script at:

   http://sourceforge.net/projects/bootinfoscript/

Then in a terminal (Ctrl+Alt+T) run command:

    sudo bash ./boot_info_script055.sh

This will produce a file named: RESULTS.txt
 Paste content of this file in http://paste.ubuntu.com/ and provide in this thread the link to Web page.
 Please don't post file directly in thread, else it will make thread difficult to read.

The magic number is 587406 for the post in paste.ubuntu. i hope this will lead to some success! Thanks for your assistance.

menuentry "Windows Seven (loader) (on /dev/sda5)" {
 insmod part_msdos
 insmod ntfs
 set root='(hd0,msdos5)'
 search --no-floppy --fs-uuid --1CD9DAF03EF31124
 chainloader +1
}

Try this. I'm little surprised to see Seven on an extended partition. Previous version of Windows had to be installed on primary partition (from sda1 to sda4).

As long as the MBR/BCD is in place where it wants in (sda1) then it can work. I'd not recommend it. If the Windows install breaks you can only nuke it (you cannot fix, repair, etc.)

This method is not recommended UNLESS you are dual booting (you have say...xp on primary 1, and 7 on a logical partition.

I.ve been out of my depth for a while - but fascinated! Now I'm a little lost. Should I not use sudo update-grub2 sometime around this point. I've put your text into /etc/grub.d/40_custom, but it looks strange amongst all those shell scripts: how does it get into /boot/grub/grub.cnfg which is where it's got to get isn't it? Perhaps rashly, I gave sudo update-grub2 a run, and it returned a complaint about the brackets within " and ". I removed those, and got a list of complaints that it couldn't find things. Am I messing everything up?!

I'm sorry, but I cannot proceed further with this at present. Reasons and apologies are in paste.ubuntu item 589655.

Sorry, but I've let this one lapse. I now am running Vista - reluctantly, and Ububtu 10.04 from a disk. This satisfies my requirements for now. I'm hoping to get a Ubuntu driver for the scanner on my machine, which will enable me to banish Vista again.