I have had a good look on the internet and I must admit that the nearest is Juglans Nigra, the common black walnut.
http://www.foragingtexas.com/2008/08/black-walnut.html

It is a black walnut tree. Look around the base of any black walnut tree, it is completely barren of any growth, no grass, no weeds, nothing and that is exactly what I see in this photo.. That is because this tree exudes a chemical that inhibits and kills all growth around it and this characteristic is called “allelopathy”. The black walnut tree in the eastern USA ( where I live) is in deep trouble and is being devastated by a disease called the “thousand canker disease”.

Were all the leaves like the one in your second photo, Rhys? It looks like it had a terminal leaflet, but it broke off at some point. The closest match that I am finding is the California Black Walnut (Juglans hindsii).

Thanks so much. I see the juglans regia a fair bit but not any others but I get it now. the barren ground around the base is a nice diagnostic to look out for.