A search for 'mustard Silvertown' online gave ...
<i>Pair of early 20th century 'CWS Mustard Silvertown' tinplate walnut effect tea caddies, litho-printed with marquetry inlay, the top with wreath and ribbon tied musical instruments, 7 in.
MUSIC CASKET. A COOPERATIVE WHOLESALE SOCIETY TIN In the form of a sarcophagus oak caddy, marked C.W.S. Mustard, Silvertown, 7in. (18cm.) wide.
tin, CWS Mustard; 12.00 x 8.40 x 5.20
tin, brown CWS Mustard Silvertown; 11.50 x 15.50 x 9.00</i>
...amongst others.
So far, I can only find the CWS Millenium (Flour) Mills in Silvertown, but the above would appear to indicate a mustard factory there as well.