20 degrees c is the equivalent of 68* F, which is not warm enough for a hedgehog. Plenty of bedding can help, but the temperature still needs to be warmer than that. If the temp is warm enough, a heating pad is not a necessity, but it is a good idea. The snuggle safe discs are ok, they are GREAT for travel, but they do not maintain an adequate even temperature, so they get cool before the advertised time to warm them up. They also make it so that you can't be gone for very long at a time, as they need to be warmed up every 6-8 hours or so. You would be better off if you raised the temp in the room up to about 23* C, that would be more comfortable and less dangerous for your hedgehog.