Hi Wally,
There are various ways to dig further. Which way do you want to go? Have you come across FreeREG yet?
http://www.freereg.org.uk/cgi/Search.plIt lets you search many Somerset parish registers. Not all are included yet, but Timsbury is. You'll find Elizabeth Bird baptised in 1826, but not Sarah or Priscilla Bird. Which is odd as BathBMD shows Priscilla Bird registered in 1843 in the Clutton sub-district, which included Timsbury. Perhaps she was not baptised. It looks likely that Elizabeth Carter was the former Elizabeth Bird, but I can't see any sign of her marrying a Carter. Have you traced her in the intervening censues? If you can afford it, it would be interesting to see what father was given on Priscilla and George's marriage certificate.
If you want to take the Slips further back, I have a suggestion. I assume you know John was the son of James Slip and Mary Brewer, both born about 1806. I believe James was baptised in Langridge, as the son of Joseph and Sarah Sleep. The family seems to use both forms of the name. Joseph and Sarah change to Slip for some of their later children; James and Mary are Sleep for the baptism of their daughter Mary Ann in 1824 at Woolley. Joseph's baptism and his parents' marriage can also be found at Langridge.
I tried to attach a photo I have of James and Mary's grave, but apparently it's too big. If you'd like a copy PM me your e-mail address.
Here's the text
SACRED
to the Memory of
JAMES SLIP
who died Feb 3rd 1831
Aged 25 Years
Through his short course, in virtue's path he trod.
His life was sober, and his actions good.
True to his God, to man he faithful prov'd.
And died in peace, lamented and belov'd.
Blessed are the pure in heart
for they shall see God
St. Matthew Chap V verse 8
Sacred to the memory of MARY GRAY
died NOV 15 1855 aged 51 years
A tender mother and a virtuous wife
David
(Woolley resident)