Your heat mat temp sounds right I believe. But is that the temp of the glass on the inside bottom or is that what it is set on with the thermostat which means the glass temp would be lower.
Your cold side and ambient need to be higher. 27 C. to 30 C. I believe is what you want. We use Fahrenheit over here so I used a conversion chart and in Fahrenheit I try to be 80 to 86 ambient.
Do you have access to night heat bulbs using a dome or ceramic heaters or even rhp's ( radiant heat panels)? Something beside your UVB bulb. Ball pythons don't need a UVB bulb and the UVB bulb shouldn't be on in the night
Whatever you use as a heat source should be on a thermostat but looks like you know this.