After a few more installs I think I've got this nailed now.
There's no need to use a live CD to delete the logical partition first. this can be done in the Debian installer as it goes along. I've come to the conclusion it's best not to use a logical partition at all. i don't know why these ever came about, I think they are probably a legacy thing from earlier computers.
The Debian installer cannot reformat the / partition whilst leaving the /home partition alone because it doesn't seem to recognise the mount points (on installing) if they are in a logical partition. you have to delete the whole installation. the only way around this is to manually create separate primary partitions on installing for /home etc. I didn't know you could have more than one primary partition.
The problem is created if you install Debian after another OS in the first partition. if you allow Debian to automatically partition the free space, it will create a logical partition. To get round this, manually create the partitions and select primary instead of logical.