If they have how likely is it that you will obtain a place. I knwo people doing MAs in different fields due to wanting to do a PhD in a different field but don't you need a diploma instead?

It depends how different the field is. I know several people who did a Master's in Gender Studies and are now doing a PhD in Sociology, and similar such shifts within, broadly said, the social sciences. I also know that there are some programs that will accept scientists into social science programs which are aimed at social science aspects of science.
If the fields are radically different however, you might have to go for the diploma. Before you do that, though, do talk to the people of where you want to go ultimately and see what they think. You might have some extracurricular expertise or something that will make them accept you directly into their PhD program.