Grape vine help

19AmyG

We have a large grape vine growing on the back of our new house. We like it and want to keep it but it seems to be getting a bit out of control! It has a lot of brown wood underneath all the green, about half a metre thick, and I suspect it has been a little neglected over the years. I wonder if anyone can give me advice on the best way to cut it back? How harsh can we be with it? Thanks.