What tree is this?

The grafting would have been done by the nursery that grew it Mark.

 

Been back to garden centre and they are on the case. He confirmed that it had been grafted when young and the bottom is def cherry and he thinks the top is also a type of cherry. I am now stuck! Why are trees grafted?? I will take a few more pics of leaves and branches in case it helps. Other than that will have to take pics when in flower in the spring. Trees don't naturally grow to a rounded shape, do they?

 

With some trees the rootstock is weak and they don't grow well so nurseries often graft them on to another stronger tree. Some trees do grow to a natural rounded shape but I wouldn't include cherries in that.