According to my early research, which was sparked off by my son Michael in August of this year, it would seem that there are an aditional 4 children before Grace.
This is based on the 1851 census and comparisons with the 1971 census.
The children shown on the 1851 census are:
Rosina, born 1844 Gwennap John, born 1846 Gwennap Elizabeth L, born 1847 Gwennap Mary E, born 1848 Carharrack Grace, born 1850 Gwennap.
William aged 29 and Elizabeth aged 26 are also shown on this census.
When this is tied to the 1871 census, it shows that William and Elizabeth nee Webster had 12 (twelve) children in total. I hope this information will be of some help to you and to any other interested parties.
I take it from your posting that Francis was your ancestor. I would be grateful if you could give me any information on this line as I have got as far as Elizabeth 1877, Edwin 1879 and Emily 1881 but nothing further at this time.