Assuming that you have normal heating and ac systems, it is going to
be far more expensive to keep them going overnight.

You should be using an optimum start controller to get the desired
temperature first thing in the morning regardless of outside
temperature.

The Carbon Trust have some good concise publications that can help,
try starting from here:
http://www.carbontrust.co.uk/
and go to publications

Also you might be able to get free advice over the phone from the
Energy Savings Trust:
http://www.energysavingtrust.org.uk/help_and_support

You might also check to see what your local council is doing to
provide help and support.