how do i find out what is causing the battery in my citroen saxo to drain?

I have aR reg saxo 1.1 expose and in July found it one morning with the battery completely dead. The battery was tested and I bought a new one. After this the radio was not working,needing a code which I didnt have and was unable to obtain. A couple of weeks later the battery was drained again! I know nothing about cars,but read through the handbook and noticed that one fuse controlled radio memory(and some other stuff) so thought I'd take it out.I did this and no more problem,
all working well except central locking etc controlled by the fuse. This week I obtained another radio,(that reQuired no code)fitted it and replaced the fuse. It has been in 2 days and I know have the same problem,completely flat battery. Can anyone give me any clue as to what is happening and what to do about it? Thanks!