Unless a specific delivery time has been agreed, the trader has an obligation to supply your goods without delay and in any case within 30 days of your order. Check the provisions concerning delivery time in the terms and conditions before you place your order.
If your goods are not delivered at the agreed time, send a written complaint to the trader and specify the date by which you expect to receive your goods.
As long as your deadline is reasonable, you can cancel your order and ask the trader for a refund if your goods have still not been delivered by that date.
We have prepared a number of templates for making complaints in different circumstances. A template for complaining about late delivery can be found here.