THE future of a unique British army unit that has served in Hongkong for 46 years is in doubt unless the Government throws it a lifeline. Guarding the British Garrison's ammunition stores on Stonecutters Island has been the job of the Army Depot Police - a unit of 32 Indian Sikhs. But, come 1997, the men face redundancy. The Civil Engineering Department has announced plans to build a new magazine at Kat Shau Wan on Lantau to replace the stores. But who will provide security has not been announced. The new base is scheduled to open in January 1997. As the Sikhs are classed as Locally Enlisted Civilians and have British National (Overseas) passports, they would be able to work in Hongkong after 1997 without having to meet the stringent Immigration Department rules imposed on former Gurkha soldiers wanting to enter the territory. Major George Ferguson, commanding officer of ammunition services on the island, said the uncertainty had not dampened enthusiasm for the jobs when the army recently advertised for another five recruits. ''Families put their sons up for the jobs as the posts are held in very high esteem within the Sikh community in Hongkong,'' he said. ''We made it very clear to the candidates that the jobs were only on a contract until 1997.'' The Sikhs were first recruited to watch the now 60-year-old depot in 1947 after a number of small fires broke out while the complex was under the guard of Pakistani-born guards. ''It was originally decided to choose the Sikhs because it is against their religion to smoke - something which is obviously very useful when you have got ammunition so close,'' Major Ferguson said. A group of men who arrived in Hongkong on a stopover from Shanghai, as they made their way back to Punjab, were offered jobs by the British Garrison. At its post-World War II peak, the 2.8-hectare depot which was carved out of the rock on Stonecutters Hill held millions of small arms rounds and naval shells for the big guns which were built on the island. Although the stocks have dwindled, the stores still have enough bullets to keep the Garrison and the Brunei-based Gurkha battalion self-sufficient under normal circumstances for a year. Inspector Sarjit Singh, who is the unit's operational commander, said he was hopeful his men would be taken on by the Hongkong Government to man the new depot. ''We certainly have a great deal of experience in this field and many of us have spent most of our lives here,'' he said. A Civil Engineering Department source said the men could be considered for jobs at the new depot. ''We would have to think very carefully about the political aspect of such an appointment,'' he said. ''China seems to be making an issue of the disposal of military sites and appointing former British soldiers may not be well received.''