Welcome to site Jessop
You've come to the right place for good python advice.
Great pose in your pic. Very alert.
Hi Dave. You said that your "thermostat [is]set at around the 25c. The heating is on a timer and is off from around 11pm till 6am. There is a low wattage heatmat about 1ft square that is on all the time and they do coil up on it at night."
I'm no expert & still have alot to learn but these are my thoughts:-
Firstly,
are you still feeding your Diamonds?
If you are, then this is a good regime, with perhaps the option for basking under a light in the morning as Serpenttongue advised, as they love to bask.
Around 30-33 oC is safe.
But if you've stopped feeding, as is normally best at this time of year for healthy adults, then I'd do away with the night heating or at least cut it back.
As long as they've had the opportunity to reach optimum temps during the day, they should be O.K.
I just reckon that they may need a longer cool night than the 7 hours you're giving them at the moment to brumate properly.
Eering on the side of caution is understandable but I've read that partial brumation uses up their energy reserves a bit more & doesn't give their immune system the full kick it needs.
I have noticed that temp at 4am is certainly down to 15c
That sounds good, but they can handle alot lower without problems (2-5oC) as long as they've got thick hollow logs or polystyrene foam insulated hides and are off the cold floor & there's no draughts.
But because you're just outside their natural range, protect them against sub-zero temps, by mildly warming the room they're in on freezing nights.
I also think it's important to keep lights off (in the room as well) and even heatmats in accordance with natural light hours.
TrueBlue strongly recommends a temp gradient as essential (& I agree) for Diamonds, so always have a cooler end if possible.
This is harder when you've got a thermostat, as they may want to get cooler than 25 oC.
I never use thermostats in cages for this reason.
Just my thoughts (for what they're worth) about what works for my Diamonds.
Of far more worth is Serpenttongues advice, he's the man. 8) Lord of the Diamonds, Prince Of Darkness :wink: