The marriage is not on FreeBMD. Marriages on Freebmd are nearly complete for the time frame 1881 (when we know they weren't married) - 1909.
The only years that are not complete are 1886, 1887, and 1888
You can view these years quarter by quarter here (shouldn't take too long):
http://content.ancestry.co.uk/iexec/?htx=List&dbid=8965&offerid=0%3a7858%3a0Tanja

Here are the 1881 details in case you didn't follow my above link:
RG11/1791 52 1
Colchester St Martinb
George StreetThomas Sargent, head, wid, 44, Gentleman formerly Coach Builder, b. Ardleigh
Sarah Ward, Housekeeper, unm, 26, Formerly Tailoress, b. Colchester
Emily Eliza Ward, dau, 6, scholar, b. do.
Thos Hy Sargent Ward, son, 3, b. Colchester