In the winter, is it more or less efficient / cost effective to leave
office heating on a low temperature over-night rather than switching
off and then having to bring the office up to a comfortable tempeature
in the morning?

Vice versa in the summer with Air Con.

My gut instinct tells me that we should switch off, but others in the
office (probably those that are 'sensitive to the cold'!) argue that
it takes more energy for a heater or Air con unit to work at maximum
power in order to acheive the right temperature in the first instance
than it does for it to run on a low out-put and maintain temperature
over-night.

Thought and suggestions please!