The wall junction is often involved
Where the bay roof meets the main wall there should be a sound upstand and flashing. Cracks in render, failed lead or a low upstand can all let rain behind the covering.
Small outlets block quickly
Some bays drain through a narrow outlet or short gutter. A handful of leaves can make a big difference. If the leak appears in very heavy rain, check the drainage before condemning the whole covering.
Timber beneath may tell the story
On a long-standing leak the deck or front timber can soften. That changes the job from a surface patch to a small structural repair as well. A springy edge or sag is worth mentioning when you ask for a quote.
Keep the detail simple
Bay roofs often accumulate layers of felt, mastic and paint. Stripping back to a clear, properly formed detail can be better than adding another layer around the edges.