Hi Leah,
Have been through the 1851 census for West Malling, but I'm afraid I couldn't find Elizabeth living there. As you say, there are quite a few Randalls living in Back Street, including an Elizabeth aged 75 born Mereworth. All other Randalls found were born West Malling.
Where does she next turn up. I'm happy to look through some other local parishes if you have any more leads.
xx Carol.