I think it's a must not only for breeding but in general. Included in there should be vitamin D3 so the calcium gets metabolized. I use Repashy calcium plus which is all in one. I'm not sure if it's available in Europe, but it's worth checking.
I too am not sure if Repashy Calcium Plus ICB is available but I know that you can get T-Rex products and the T-Rex Leopard Gecko ICB is the same stuff.