I have been researching William Langman and his family on and off for a few years now, and I too have come up with precious little on the Langmans and Gilberts in their homeland.
One interesting thing that my research points to is that William Langman was from Devon, not Cornwall. The marriage record in the St Hellier registery states that Elizabeth Gilbert was from Callington and that William Langman was from Plympton St. Mary, Devon.
Another interesting snippet is an old bit of family lore related by a Mrs Margaret Walters (and passed on to me verbatim by Rex Vivian a few years ago in a letter responding to my enquiries about the family) which says that she heard from "an elderly aunt" that "William was born in 1809, and that the midwife was a Nurse Pointer, who reared William from birth, until his early teen years. Nurse Pointer had a son who was living and working in the Channel Isles. Our William was sent over to the Channel Isles to live with her son. The son had a business. He was a white smith. Apparently William learned this trade from him."
To my surprise I found the Pointer family really did exist, being recorded as a family of English origin living in Jersey in the 1841 Census. They were the only family of that name living in the Channel Islands, and they were smiths (there were two Pointer brothers, George and Benjamin, both aged c.50, and the right age to have had William apprenticed to them over a decade earlier). I found a George and Benjamin Pointer of their ages in the Plympton St. Mary registers, the former being recorded as a blacksmith on his daughter's christening. This family also disapears from the local records, presumably because they have shifted to the Channel Islands. Although all this lends credence to Mrs Walter's second hand account, I have found no trace of William Langman in this parish.
Also, I have two photos of William and one of Elizabeth which I would be happy to send to you (either by email or post) if you do not have them.
My G Grandmother was Edith Louisa Maud Langman (1874-1968), the eldest daughter of William's son John (1844-1909). Edith was estranged for many years from her family in Adelaide, so I would be very interested in any stories, photos, etc. which you might have about the Langman's in Australia, and particularly John's family.