Don't rely on the temperature markings on the thermostat's dial. They are almost always incorrect - they are on every single one of mine. By this I mean that if you set the dial to 80'f, then this doesn't ensure that the temperature in the viv will be 80'f.. the dials just aren't accurately calibrated on what are relatively cheap thermostats.
What you need to do is buy a digital thermometer. You can pick these up off ebay for a few quid. Place the probe of the thermometer in the warm end under/over the heat source and then see what the reading is. If it's higher than you are aiming for, then turn the dial on the thermostat down a fraction and then leave it for 20 mins before repeating the process. The thermostat dial should be used in this "up a bit, down a bit" manner, and will result in an accurately controlled temperature. But just don't rely on the temp scale printed next to the dial - they lie.