If you can keep the room that the tank is in at a steady 24-26C, then you will be ok, but remember that temp. fluctuations can upset sensitive fish and increase vulnerability to illness. A small amount of water will fluctuate further and more easily than a large one when there are ambient temp changes. I think a heater is best to be on the safe side. Can you set a room heater thermostat to keep the room temp stable? Is there a lot of traffic in/out of the heated room (ie door opening constantly=more temp changes). If you are really desperate, you could try putting the tank against a very solid (ie double brick or similar) internal wall that is as close to the centre of the house as possible, shut all doors and windows into that room and hope it keeps your temp more stable.
ps please bear in mind that i live in a totally different climate to you, so some info may be different!!