diff --git a/live-build/ubuntu-cpc/functions b/live-build/ubuntu-cpc/functions
index ee254d4b..3c8c7853 100644
--- a/live-build/ubuntu-cpc/functions
+++ b/live-build/ubuntu-cpc/functions
@@ -52,12 +52,8 @@ mount_image() {
         exit 1
     fi
 
-    # don't assume we know the separator between device number and partition
-    # number in the partition name, as this has changed over time.
-    partsep=$(echo "$loop_p1" | sed -e's/^loop[0-9]\+\(.*\)[0-9]\+/\1/')
-
     # Find the rootfs location
-    rootfs_dev_mapper="/dev/mapper/${loop_device}${partsep}${rootpart}"
+    rootfs_dev_mapper="/dev/mapper/${loop_p1%%[0-9]}${rootpart}"
     if [ ! -b "${rootfs_dev_mapper}" ]; then
         echo "${rootfs_dev_mapper} is not a block device";
         exit 1
diff --git a/live-build/ubuntu-cpc/hooks/032-disk-image.binary b/live-build/ubuntu-cpc/hooks/032-disk-image.binary
index 13c197d9..1b6d3099 100755
--- a/live-build/ubuntu-cpc/hooks/032-disk-image.binary
+++ b/live-build/ubuntu-cpc/hooks/032-disk-image.binary
@@ -59,7 +59,7 @@ mkdir mountpoint
 mount "${rootfs_dev_mapper}" mountpoint
 
 if [ -n "$BOOT_MOUNTPOINT" ]; then
-	boot_dev_mapper="${rootfs_dev_mapper%[0-9]*}1"
+	boot_dev_mapper="${rootfs_dev_mapper%%[0-9]}1"
 	# assume fat32 for now
 	mkfs.vfat -n system-boot "$boot_dev_mapper"
 	mkdir -p "mountpoint/$BOOT_MOUNTPOINT"