You may find it helpful to look at some of the help pages on
www.hertfordshire-genealogy.co.uk which has information on where you might find information prior the the start of civil registration in 1837, and may also give you some ideas of where you might look if, for example, you ancestor was baptisted in a non-conformist chapel whose registers have not survived. At least with a birth in 1835 there is a chance you can identify the parents from census data.