Hi James -
Welcome to Kundalini 101!
Snakes in dreams tend to symbolize either small fears that we have - items that can reach up and strike us when we are not looking out for them - or they, more commonly, symbolize the life force - or what is often referred to as Kundalini.
In Indian Yogic teachings the life force of the body lays coiled at the base of the spine - like a snake - in the first energy chakra. There are seven energy chakras (centers) in the body, ascending from the base of the spine up through the genitals, solar plexus, the heart, throat, third eye and then the crown chakra at the top of the head.
When our Kundalini or life force is awakened, the serpent begins to crawl up through the chakras - headed for the crown chakra - but without any guarantee, really, of reaching it. This is because there are plenty of places along the way where one’s energy might get diverted or stuck.
We may become over-concerned with sex or material power, for example, and then our energy never would rise above the third chakra - the solar plexus. Once the serpent is able to reach the heart chakra (# 4), however, the rest of the journey is fairly assured. If we can learn to see, hear, and feel with our heart, then we are on an infallible path to enlightenment.
Your dream is extraordinary for its clear symbolic depiction of the rising kundalini (serpent) energy. The snake enters your trousers and then crawls up your body. It is stuck momentarily at your waist - chakra #2, sexual energy - but you make an adjustment and loosen your belt and allow the snake to continue on its path. The snake then exits at your neck - fifth chakra - which is short of the third eye and crown chakra but significantly above the heart.
In most spiritual traditions, your dream would be considered a positive harbinger. You are “normally horrified by snakes,” but in this dream you allow the snake to come into very close contact with yourself and you are not frightened. You allow this process to occur. In a simpler interpretation, one might say that you are allowing some of your fears to gain conscious access and expression - always a healthy process. A more symbolic interpretation confers a status of psychological wisdom and maturity upon you. You are allowing your own intuitive, psychic, and energic sensitivities to develop.
A search on the Internet, under keyword "kundalini" will prove interesting.