I have an 18 week old Beagle dog and finding it very difficult to toilet train him. He is in the garden most of the day but as soon as I let him in he does it in the house. Can anyone give me any advice?

You need to be out in the garden with him all the time to tell him your word of command when he starts to perform. Otherwise, if left to his own devices he'll think it's okay to perform wherever he happens to be, whether it's indoors or outdoors.