The room I use for fermenting is rather cold currently at around 8 degrees C, I had the idea to use a aquarium heater in a box of water to keep the temp around my bottles around 18 degrees, my aquarium heater arrived and its lowest temperature is 20 (strange it said 16 on the box its 50W), is this too high for second fermentation? The recommended temperature is around 18 so i've read. Will I cause any damage setting the temperature to 20? I am brewing a bitter and trying to follow the instructions with this brew to see how it turns out from the instructions as this is my first time home brewing.
You wouldn't want a sudden drop in temperature, or too low (8C is way too low), the yeast might give up completely instead of helping the beer mature. 20C should be no problem, the yeast shouldn't be making any esters at this stage. It's also the preferred temperature for the early stage of bottle conditioning.
This all assumes you're using a common ale yeast, and nothing else.
Since there is really no fermentation going on in secondary, temperature is of little concern. as log as you don't get too hot, not much else matters. In fact, there's seldom a need to do a secondary at all.