HeHe I also have UK SAmKnows WRT54GL, haven't broken into it - portscan reveals its running OpenWRT, and still reporting throttled performance back to ISP! The flash space used by the kernel won't appear in that list.
One way of finding out how the flash is partitioned is to look at messages during boot, which should still be available later using "dmesg", an 8MB flash old wrt54gsv1.1 here reports
Physically mapped flash: Found an alias at 0x800000 for the chip at 0x0
Physically mapped flash: Found an alias at 0x1000000 for the chip at 0x0
Physically mapped flash: Found an alias at 0x1800000 for the chip at 0x0
cfi_cmdset_0001: Erase suspend on write enabled
Using buffer write method
Flash device: 0x800000 at 0x1c000000
Creating 5 MTD partitions on "Physically mapped flash":
0x00000000-0x00040000 : "pmon"
0x00040000-0x007e0000 : "linux"
0x000be800-0x00340000 : "rootfs"
0x00340000-0x007e0000 : "jffs2"
0x007e0000-0x00800000 : "nvram"
sflash: found no supported devices
but note the addresses for rootfs and jffs2 are within the space "linux"
# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 2.5M 2.5M 0 100% /
tmpfs 14.9M 204.0K 14.7M 1% /tmp
/dev/mtdblock/3 4.6M 508.0K 4.1M 11% /jffs