Hi Graham
Only just seen this post. I have connections to Polperro which stands between the parishes of Lansallos and Talland. Although I only have distant connections to the Langmaid family I do have info passed along from others. As Graylight says, if you havent been in touch with the Polperro FHS then you should (I'm a member) they have so much info.
What I have on 'your' John Langmaid is :
Born c1755 married Rebecca Sweet.
Children : George 1791, John 1794, Rebecca Sweet 1802, Joseph 1804 & Mary 1806
Parents : John Langmaid b c1723 Elizabeth Goard 1725
Siblings : Catherine 1748, Mary 1759, Rebcca 1761(two other Johns b1750 & 1754 both died)
Have various bits on other Langmaids that may or may not be connected to you

Fiona