Hi Peh Lin,
I have the same problem as you. You don't need to talk to your neighbour as HDB officer will settle everything for you. All you need is to agree to reno and pay for it. If your neighbour refuse, HDB will take warning action. For my case, I'm really angry with my neighbour living downstairs. You know what, during my reno(for nearly 2mths), they never said a word about their ceiling leak, only when everything is done and I have moved in for about 1 mth. My hubby was really pissed off, well we agree to let HDB to reno the toilets for us then we came to know that two toilets take about 8 days to complete. Ok, that problem still can be solved as maybe I can get my mum to stand by for me. But,my only headache is where shall I put my furniture and bed during the reno. When I think of the reno works going on, my head burst. But despite of all these problems, my neighbours have nothing to gain as they will only get their problem solved because HDB will not plaster their ceiling and only will do our toilets floor tiles. Anyway, for just $400, I can get new floor tiles for both toilets, its still worth it. (Hahaha, lucky I didn't reno my toilets)
Hope your case can run smooth.