Vacuum cleaner hoses come in a variety of diameters, best way of doing it would be to take your hose into a local vacuum repair shop and ask the question. The DC18 is long discontinued and there are no generic hoses available.
The part number for the hose is 91169404, Googling it brings up a couple of possibilities of a new one and there is an Ebay entry for a used one. The DC18 is about fourteen years old now and is still worth spending a few pounds on as it’s much better than the current crop of machines.
Don [h=1][/h]