Ford KA water temp light when idle

Hi need some advice as my partners water temp light has started coming on but only when idle.

Here's what happens

Start driving fine then after a little while we stop at lights the temp gauge starts to increase until it hits the red and the light comes on again. As soon as we start moving it drops down to the normal range until we stop again and it jumps back up!

It's a 61 plate.

Any advice would be great!