LONDON - British troops yesterday began moving from central Bosnia through Serb-held territory towards the isolated Muslim enclave at Srebrenica, writes Christopher Bellamy. UN sources said their deployment had been delayed by negotiations with Serb forces. The troops are to guard Srebrenica for three weeks as a stop-gap measure, relieving Canadian troops until a permanent replacement can be found. Srebrenica has been cut off for months and very few aid workers or journalists have reached it. The UN commander in Bosnia, Lieutenant-General Francis Briquemont, asked the British for help because the Serbs had held up equipment for the Nordic battalion, which was due to protect the enclave.