Ideally, as I'm sure you know, the viv should be set up at least 48 hours beforehand to make sure the temps are ok. As this isn't possible for you then this is what I would do: If it is an adult it will probably be ok for a few hours without heat at this time of year. If it is a baby I would put the Leo in a cricket tub (paper towel on bottom) and then put the cricket tub and Leo in a cool box or poly box which has got a hot-water bottle (not too hot and wrapped in a tea towel) or heat pack in it. As soon as you get home get the viv set up and wait for the temps to settle. If this is a second hand viv then hopefully the thermostat will be set so that the temperatures should be somewhere near what they should be.
For a Leo a heat mat is a much better method of heating than a heat lamp imo - but I'm sure some ppl will disagree. Good luck with your new Leo - don't forget to post pics!