Brian,
I was too focused on "Walrus" that I forgot that your question was in relation to a Do-18!
Anyways, the lost Walrus of 24-4-1940 is mentioned on this website as well:
http://www.naval-history.net/xDKWW2-4004-13APR04.htm
No mentioning is made here of being shot down, but it adds that it operated from the cruiser HMS Southampton.
Regards,
Leendert