Which kit are you using, the bavarian lager and pilsner are both supplied with lager yeasts which will work at lower temps. If using the ale yeast that comes with the lager,real ale,stout, pale/sparkling ale ect. then 15 C is a bit low. It will take a couple of weeks to reach final gravity. Try wrapping the fermenter in a sleeping bag/blanket, fermentation generates a small amount of heat. I think that maitaining 18 C will esure complete fermentation. Might be a good idea to choose a yeast that is comfortable at the temps you plan to brew in.
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Cheers