I believe it is Tilia cordata. Small leafed lime. http://images.google.com/images?hl=...-GB&q=tilia cordata&um=1&ie=UTF-8&sa=N&tab=wi http://en.wikipedia.org/wiki/Tilia_cordata
Since the twigs appear yellowish it might be a yellow-twigged cultivar. Such distinctions are more apparent during winter.