Seek and you will find

Well, it's taken an age and a lot of poring over internet records, joining different forums and family history sites, and sending emails to all points of the compass, but I finally made the breakthrough tonight.
I now have the evidence that links my great great grandmother to the Harvey family.

One of the problems in internet research is that you rely on other people reading 150 year old handwriting correctly, plus the non-standard spelling of family names. Many of our ancestors were illiterate, and there was no standard spelling of names and places, so sometimes you just rely on luck.

I have been lucky because I have a copy of the family bible, and so have exact dates. I've also been helped by the online parish clerks responsible for updating the records. I've been able to send them information to correct the online entries, so aiding the next searcher.

To be honest, I was stumped. I could find no trace of the Nicholas Harvey family in 1861. I had them in 1851, in Hayle, but my ancestor would have been 11 at the time.
I did find this entry
http://freepages.genealogy.rootsweb.ancestry.com/~sterth/1851_ed1a6.htm

Hayle Foundry 55
Harvey Nicholas Head M47 Manufacturing engineer Helston
Harvey Julia Wife M 30 Prussia [sic]
Harvey Sophia Dau 11 mos St. Erth
Richards Mary Servant U 27 Cook Germoe
Rowe Ann Servant U 21 Childsmaid Perranuthnoe
Simions Elizabeth Servant U26 House maid St. Austle

You will note a daughter Sophia aged 11 months.

I found these entries from 1852

http://freepages.genealogy.rootsweb.ancestry.com/~phillack/strays/st_erth_bur.htm

Harvey Juliana Henrietta Hayle Foundry 9-Jul 1852 8 m
Harvey Nancy Sophia Hayle Foundry 22-Jul 1852 2

So Nicholas and Julia Harvey lost two daughters within a few weeks of each other. Sophia was 2 and her younger sister was just 8 months old. We forget just how many children failed to live beyond the age of five until quite recently. Infant mortality was a major problem. My father had an older brother named David who didn't make his first birthday. That was in the 1920s.

So, did the fact that they lost their first two children influence their appointing my great great grandmother as nurse/governess?

I looked everywhere but could not find a trace of Nicholas and Julia Harvey in the 1861 census, so I emailed the Online Parish Clerk for Phillack, one John Smith who lives in Australia.
He replied within a day to say that "I have found Nicholas HARVEY and his family at the time of the 1861 census. Nicholas and his family were living at 13 Ladbroke Terrace in Kensington (London). "
Unfortunately the pdf he attached did not list the servants, so I had to dig around a bit more.
Once again, a search of the names yielded nothing, but a search of the address hit paydirt. The surname had been copied incorrectly and read "Hawey", but the details were enough to establish that this was the family I'd spent hours searching for.

1861 Census
224 HARVEY, Nicholas B Head Married M 57 1804 Engineer Helston Cornwall
224 HARVEY, Julia Wife Married F 40 1821 Prussia
224 HARVEY, ... Daughter F 5 1856 St Erth Cornwall
224 HARVEY, Henry N Son M 3 1858 St Erth Cornwall
224 HARVEY, William L Son M 2 1859 St Erth Cornwall
224 HARVEY, Fransis H Son M 1 1860 Kensington Middlesex
224 HARVEY Son M 0 1861 Kensington Cornwall
224 HARVEY, William Visitor Widower M 56 1805 Merchant Helston Cornwall
224 HARVEY, Charlott F Visitor Widow F 61 1800 Lady Helston Cornwall
224 MARSHALL, Elizabeth Nurse Widow F 59 1802 Monthly Nurse Cambridge...
224 WILLIAMS, Mary B Servant Unmarried F 25 1836 Cook Mawgan Cornwall
224 ROW, Angeline Servant Unmarried F 29 1832 Housemaid St Hilary Cornwall
224 VALMER, Fredericka Servant Unmarried F 22 1839 Upper Nurse Prussia
224 MARLAM, Margaret Servant Unmarried F 18 1843 Under Nurse Liverpool

At last!

Now for the spooky part.
The Harveys moved to London from Hayle and were living there in 1861.
In 1962 my family relocated from Hayle to North Kensington.

The Harveys lived in Ladbroke Terrace in what is now a very desirable part of Notting Hill. It's just around the corner from Ladbroke Grove. If you travel north along Ladbroke Grove for a mile or so and turn left by the railway bridge, there's a small cul de sac of houses called Barlby Gardens, where we lived for many years.

That's spooky.