It's none of those, the flowers are separate and delicate and shaped like laburnum but in a different arrangement - there's one of these trees in the gardens of Southover Grange in Lewes.
I agree the flowers looked like a conker tree, but the leaves are wrong...I have seen this before also, but in some botanical gardens somewhere. The leaves on this are similar to maple but are more triangular, where as a conker is like a sausage shape