If you are using the single 7g packet of kit lager yeast you might struggle to get it to ferment at 13*C. I did the Euro lager which comes with the same yeast and it sort of died so I had to raise the temperature. However if you read the Forum reviews on the Coopers Pilsner and Euro lager kits others have had more success at low temperature. So my advice is that unless you are going to add more lager yeast I would leave it for say 24 hours at 21*C to get a good crop of yeast going then allow it to slowly cool but keep an eye on it.