Wisteria is wilting

Please advise me how to save my Wisteria-its established & growing against a porch wall & up along the front of house house & along wires. The roots are covered with purple slate as is the whole bed with only small plants in it & another climbing Hydrangea against the opposite wall of my neighbours porch either side of a bay window about 6feet away that was planted at same time. It's now about 15 years old & only started flowering when it was 8years old or so. It's a Black Dragon variety so not the norm. It flowers sparsely & with some leaves already on the plant, different to most varieties. It's been fine for years through wind & hot spells but it's just started wilting & leaves dying & dropping off. I have watered it on & off through hot spell & dumped a bucket of water on it last week but no difference!? The edges of the leaves are dry & dark green. Is this from sun/wind burn or lack of water or frost? Or does it have a disease? It was pruned back pretty drastically by a gardener I thought was capable & qualified last October. Has he damaged it & left it open to frost damage?