Hi Bollie. Is this the original Grundig with the four pins holes on the front? If it is, you need to bend some stiff wire into two, U shapes and use these to depress the release clips. All four clip need to be pressed simultaneously. I used a welding rod but I have heard that it can be done with two people and four cocktail sticks. A photo of the head unit might help. Also, if you have a contact at an In Car Entertainment specialist, Halfords etc., they have ALL the tricks needed to remove radios. The pins/clips/barbs etc., securing the head unit are really lightweight and don't need a lot of force to push them to one side.
Steve