The surname Canham pops up a lot across the Tendring Hundred. I'm transcribing lots of those parishes for
FreeREG and I've also put them as PDFs on my website:
Essex & Suffolk Surnames - so I've seen the name appear quite often. My great-uncle was married to a Canham at one point, too. As you're not sure where they're from, it might be best to do a soundex search on FreeREG (as you can imagine, the surname is rendered in various ways... Cannum etc). Bear in mind the coverage isn't complete yet (and in some cases, never can be, thanks to lost registers or smudged handwriting!), but it might yield dividends from a quick search.
If you have an Ancestry account, you might want to have a look at the non-conformist registers too.
There are Canhams (off the top of my head) in Beaumont-cum-Moze, and there is a legend about the beautiful but consumptive Kitty Canham, bored wife of the Thorpe-le-Soken vicar, who ran off with an earl...!
PS: I've noticed sometimes people saying they were born in Elmstead being baptised in Wivenhoe. You might want to check the register on
http://seax.essexcc.gov.uk/ yourself, but I'm in the process of transcribing the register for St. Mary's as well. There are Andrews in Wivenhoe in the late 1600s. I've only got up to the 1730s at the moment though so perhaps I might find your Sarah soon.....
