You are a bit late for this year unless you have a frost free greenhouse or warm window cill

The big problem with sowing seeds at this time is not so much getting the heat right,but getting sufficient light!

To cater for this situation I wouldn't prick them out until spring so that the disturbed seedlings get off to the best possible conditions i.e. when light levels are improving!

This article might help;

http://www.thegardenersalmanac.co.uk/Data/Alstroemeria/Alstroemeria.htm

BTW Alstroemeria have pretty flower but are thugs as far as plants go!

They spread like mad and are difficult to get rid off should you want to.

It took me two or three years to get rid of mine!