Some notable events:
April 2015 There has been very little rain this month and not that much this year.
April 15th: Hottest day, 26°C, since last summer.
April 16th: For the first time the ash are leafing at the same time as the sycamore. Ash is usually a few weeks later at least (last to leaf, first to shed). It seems all the trees are leafing at the same time, which is extremely odd (ash, oak, hawthorn, sycamore). So it seems they are primarily sensitive to temperature not day length.