Luckily, Bubbles had run back into the house, and returned with their Flower Book and looked up the entry for Snowdrops. He proceeded to read out some very interesting facts, the first of which seemed to answer Richard's question:
Snowdrops come up every year and may multiply and spread over time. Once you have a patch started, they will spread on their own by self-seeding.
The flower is formed in the bulb the previous March and waits nearly a whole year before pushing through the soil.
On a sunny day, snowdrops are highly scented and give off a honey smell. If you have enough plants, the perfume will fill the garden.
Snowdrops contain their own anti-freeze proteins which is why they can survive the very cold weather.
Snowdrops are very small plants that don't have much impact when grown singly, so their bulbs should be planted closely together in groups of at least 25. If you plant them in Autumn, they will flower the following Spring.