BertieB, you may find that your child will only fall asleep well in a swaddle, as my second one did. It made a huge difference for me for him to sleep 3-5 hours swaddled vs. 1-2 hours not swaddled as a newborn. I swaddled for three months or for as long as he could still fit snuggly in a swaddling blanket. By May, I think you'll find it too warm for a sleeping bag; in fact, am pretty sure that you'll be using the aircon by then. We set the airon to about 25 degrees. Mine would sleep in a onesy and diaper throughout the summer - which is technically May to November here; winter came a bit early this year. By Dec, you may be comfortable using a blanket instead.
Are you considering a tog because of SIDS? For that, I would recommend the Graco Angelcare Movement Monitor. I don't know if you can get it here or not as I ordered it from the States, but it gave me real peace of mind. Basically, it'll alert you if the baby doesn't move for 20 seconds, and I've never had a false alarm.