Hello. Drill your holes , pack with Sodium Chlorate, it is still available, and cover the stumps with a double layer of polythene, dustbin liners are ideal for this, and tie the polythene firmly in place. About six weeks later remove the polythene and repack the holes with the sodium chlorate. Replace the polythene and leave the stumps to rot. Some may say that repeating the process is un -necessary but I have always found it beneficial. Good luck. Meanwhile keep a close watch for any seedlings, sycamores are notorious for these!