Does a Creda w100fw washing machine have a built in heater?

Yes they all have a built in heater, BUT,. There are 2 connections , hot and cold must be connected for the machine to work. Red (Hot) and Blue (Cold) must be connected to hot and cold taps . Heater needs both connected to operate or you will get only one temperature , extremely Hot which would ruin most of your clothes except towels and cotton bedding. Also the water would run out of open connecter at back of machine

Yes it has a heater. Most, if not all modern machines are cold fill only. However I connected my hot water to our machine to save on heating costs, because electricity is more expensive than gas. It only works because we have a combi boiler and no water tank, so the hot water has almost as high a pressure as the cold. It doesn't save a huge amount because a modern machine doesn't use much water. But the water heats the drum a bit so the built-in heater has less work to do, making the full wash cycle shorter.

Most new machines are cold fill only and heat on entry

You should only have a blue connect pipe if its cold fill

If your not sure refer to your handbook

In fact you should have checked this before purchasing it