OK, first let me say, I don't have any diamonds, I only have coastals but as their range overlaps considerably, it's my opinion that a similar setup can be used. What I do is:
Spring / Summer / Early autumn: Heat is on 24/7. Thermostat set to 30C. No artificial lighting provided apart from room lighting. Next year, I will be looking at turning all heat sources off at 10pm or so and back on at 6am or so. This will probably only cause cage temps to get down to approx 28C because they will be in a room with no aircon. This year, they had airconditioning so I felt it was best to leave the heat on as it gets down to 15C or so at night.
Winter cooling period: 2 weeks after last feed, start dropping temps so that over 2 or 3 weeks heating is essentially turned off, ie thermostat is set to minimum setting 7C. After a few weeks, I turn the heaters off. I could just leave them on if I was concerned about the temps getting too low. Now, if I leave the windows closed, it never gets below 13C in the herp room and about 15C in the cages. If we go away for a few days and the fire is not lit, it may get colder than that. I personally don't think that that will do any harm to either coastals or diamonds.
The advice I was giving to harty was based on the fact that he lives in melbourne and on the climate down there. If you lived at Glen Innes, I would give diferent advice. Also, if the snakes are kept in a shed, I'd also change the advice.
Of my 3 coastals (well one is an intergrade,) one is far more active with temps down around N15C D18C than she ever was in summer. One is about the same and one is almost never seen. The active one is even seen in her water fairly regularly which she never did in summer.
Also, please note everyone, this is for coastals and diamonds only. If I had them I think I'd use something similar for MD's and Bredli but not for any other python.