Leaky chimney flashing

Hi everyone, I've got some damp patches appearing on an upstairs chimney breast which I've traced back to a leak near the chimney.

The chimney was repaired for exactly the same reason 7 years ago. The slates were lifted and new lead flashing was fitted around the chimney. The stack itself was re-pointed and had a new mortar cap put on it too.

So you can imagine how annoying it was to see the same leak appear recently, having been convinced that we got shut of it 7 years ago!

I've attached a picture of the chimney. My theory is that water is getting in under the small slate at the bottom left of the stack. Right is west and it's quite an exposed spot.

Does the lead and roofing look correct? It was done by a trusted builder but surely it shouldn't be failing after 7 years?

Does anyone have any ideas how to diagnose further?

Image of the same chimney - different angle

OP,
1. Was each step flashing turned 25mm into a mortar bed chase, & then sealed with sealant?
2. Step flashing is best installed using lengths of lead - not individual pieces.
3. The ridge to stack abutment flashing should be a saddle flashing that goes under the ridge tile - was that how yours was done?
4. Are there individual soakers under each slate - soakers that have 66mm upstands?
5. The flaunching looks to be very thin - it should be much deeper/heavier.
6. Why not go into the loft & look for water stains on the loft chimney breast? Post pics of what you see, & a pic looking up at the felt below the flashing?
7. Can you post a pic of the other side of the roof/stack?