The last few lines of grldr are an "internal" or "embedded" menu.lst, they are read when the grldr is chainloaded and they - among other things - try to find the "real" menu.lst, here they are from 0.4.6a-2018-06-12:
title find /menu.lst, /boot/grub/menu.lst, /grub/menu.lst
if "%@root%"=="(ud)" && calc *0x82A0=*0x82b9&0xff
if "%@root:~1,1%"=="f" && find --set-root --devices=f /menu.lst && configfile /menu.lst
find --set-root --ignore-floppies --ignore-cd /menu.lst && configfile /menu.lst
find --set-root --ignore-floppies --ignore-cd /boot/grub/menu.lst && configfile /boot/grub/menu.lst
find --set-root --ignore-floppies --ignore-cd /grub/menu.lst && configfile /grub/menu.lst
if your menu.lst is available in the searched devices/paths, then it will be loaded, if it isn't it won't and you will have the grub4dos command prompt only.
OK, thanks, I'll try mounting them with mountvol or diskpart to see what's on them.
So it's probable that there's no BOOTMGR or boot\BCD anywhere at the moment and grub4dos can't boot WIn10 without them? I guess if I've got the partition mounted in Win10 I could use EasyBCD to create the required files on it?
I would rather use BCDBOOT.
So with the ESP partition being (hd0,1) should I just use this command?
umbr -d=1 -p=3 (hd1,0)/grldr
What purpose do the -d=1 and -p=3 bits serve, when the destination for grldr is set by the last part and what will happen if grldr can't be located after installing umbr?
Yes and no (mostly no).
Remember that you need to have grldr on the ESP partitions before running that command.
This is the translation (Google translate) of the parameters:
Umbr [-d=D] [-p=P] [--test] [file1] [file2] [file3]
-d=D Specifies that the disk to be installed defaults to the current ROOT disk
-p=D The boot partition defaults to the first partition.
--test test mode, does not write to disk, add this parameter will enter the installation effect test.
file1-3 can specify three boot file locations to prevent startup failure.
Note: This filex can Is any file format that GRUB4DOS can recognize (must be stored continuously).
For example (hdx, y) / path / file or (hdx) xxxx + yyyy / (hdx, y) xxx + yyy, etc. file1-file3 must be in the same
The disk is consistent with the disk specified by -d.
File1-2 can also be a PBR such as (hd0, 3) +1 file1 is the main boot file, if the test fails, try file2 again...
so i would try to supply ANYWAY the --test parameter, but you want to run:
umbr -d=0 -p=1 --test (hd0,1)/grldr
the -d=0 -p=1 actually means AFAICT "should any of the files linked to ([file1] [file2] [file3]) fail, try chainloading the bootsector of the drive 0 partition 1", it has to be seen if your ESP partition has a valid bootsector chainloading the "normal" BOOTMGR.