Hole in my ceiling from Chimney leak

We moved into a new house only to discover a hole in the ceiling of the airing cupboard. This was caused from a drip (once per 2-3 seconds in heavy rainfall). It may be important that the drip seems to be falling from the centre. I have had a roofer since recovering my chimney and said that it should be now water tight. He also said that the rest of it looked fine (i.e. the brickwork, flashing etc).

However, after the rain from last couple of weeks, the drip has not gone away. There are a few issues which I cannot wrap my head around so will try my best to explain it properly.

Our house is from 30's, our chimney is shared with our neigbour and has four holes (2 per side). Both ours is sealed with lead and concreted over. One of theirs is capped with the cowl and the other apparently is just open according to our roofer. However, when we were trying to look at the problem (roofer looking down my chimney and me looking up, we couldnt see each other as something half way up was blocking the view. Its hard to describe, its like a sheet of material but is semi transparent (can see daylight semi-coming through when my chimney hole was open and then went dark when we covered it again. He then tried dropping some bits of brick/mortar down and it did reach to my but it didnt sound like it came straight through and also was some time delay, albeit minor.

If it's not my chimney, could it be that water is collecting in my neighbours and then seeping through the brickwork to my side. Collecting at the middle of the mysterious material and dripping from there?

I have tried drawing a sketch of our chimney layout and was wondering if anyone can help in anyway at all!!!

 

photo of material

 

anyone?

 

Sorry, only just seen this but sadly no, I have no ladder reaching the roof. I will try and get one from the previous roofer who did the job. He did say that the brickwork and flashing was fine along with anything around it.

It happened over the weekend and I went to my loft and checked every corner fo the floor and ceiling (roof) and couldnt see a sign of water