I've now managed to solve this:
I created a live Linux Mint 17.3 USB using Startup Disk Creator, which booted successfully. The boot process wasn't entirely straightforward - I encountered an error message "vesamenu.c32: not a COM32R image". At this point you have to key "live" (without inverted commas) and enter.
From the live system, I deleted the first 3 disk partitions (details in 1st post) in GParted and created three Ext4 partitions - two of 25GB and a large partition occupying the remaining space to be used as a shared data partition.
I installed Mint as the secondary OS in /dev/sda2, booting from dev/sda in Legacy mode.
I then went through stage 1 again with LL3.0 and istalled it in dev/sda1, again booting from dev/sda.
Sorted! I hope this helps anyone with the same problem.
UPDATE:
Unfortunately, it seems I haven't solved this after all. I experienced what seemed to be an unrelated problem - I couldn't install updates. I eventually came to the conclusion that this was caused by the installation, as I could find no other explanation and I had the same problem when I installed LL2.8 in place of 3.0. I have now reinstated the original partition structure and installed Linux Mint on the laptop, as it supports UEFI. I still use LL as the primary OS on my desktop PC.
I created a live Linux Mint 17.3 USB using Startup Disk Creator, which booted successfully. The boot process wasn't entirely straightforward - I encountered an error message "vesamenu.c32: not a COM32R image". At this point you have to key "live" (without inverted commas) and enter.
From the live system, I deleted the first 3 disk partitions (details in 1st post) in GParted and created three Ext4 partitions - two of 25GB and a large partition occupying the remaining space to be used as a shared data partition.
I installed Mint as the secondary OS in /dev/sda2, booting from dev/sda in Legacy mode.
I then went through stage 1 again with LL3.0 and istalled it in dev/sda1, again booting from dev/sda.
Sorted! I hope this helps anyone with the same problem.
UPDATE:
Unfortunately, it seems I haven't solved this after all. I experienced what seemed to be an unrelated problem - I couldn't install updates. I eventually came to the conclusion that this was caused by the installation, as I could find no other explanation and I had the same problem when I installed LL2.8 in place of 3.0. I have now reinstated the original partition structure and installed Linux Mint on the laptop, as it supports UEFI. I still use LL as the primary OS on my desktop PC.