I'm also a newbie to Lenormand. Am I right in thinking that no definitive set of "rules" for using Lenormand decks actually exists?
There are "rules." The most traditional way of reading the cards are outlined in a sheet commonly referred to as the "Philippe Lenormand."
This is the trunk of the Lenormand tree, and was published with 95 % of Lenormand decks in North Europe (Germany, Belgium, Holland, France, et cetera.) until after WWII in and was translated into English and published in the States.
It was the basis for the French poems which were largely copied in France and Germany from a Belgian deck, too.
There are however several branches which come off this sheet that have some differences.
Most are minor and you can clearly see the PL sheet in them i.e. Iris Treppner or Mario dos Ventos acknowledge the Snakes’ negativity but also that she can help you in bad situations, which in the PL sheet is what happens when the Snake is “far away” (takes misfortune away). Erna Droesbeke puts the word "work" with the Moon, as in the PL sheet the Moon symbolises fame, honours and prosperity.
However, others owe more to culture and schools of thought such as the Whips as sex or the Fox as work. These are not backed up traditionally and owe more to do with people's take on a symbol or culture/literature (Fox as work seems to be based on a 1920s and 30s book popular in France and parts of Belgium then).