Chimney still leaking despite reflashing and coating
Water gets into a chimney by one (or more) of five ways:


Via the actual opening of the flues
Yours have metal covers, so this is probably okay.
Via missing, cracked or deteriorated flaunching on the top of the chimney
This is very common as the mortar has a very hard life. Your right side flue isn't sitting vertical which means it's dropped and unlikely to have a good seal to the flaunching.
Via the brickwork itself. 
Mortar erodes to the point where perpends or cross joints are non existent. Water can run in and down such erosion. Difficult to tell what condition your is in from the pic. Plus the chimney is a poor design with very little oversailing to throw the rainwater away from the main body of the walls.
Via the brickwork to roof connection Usually metal flashing.
Your's look to be in a bit of a state, repair on top of repair is unlikely to be effective. Plus, it looks like the long horizontal section looks to be in one piece. If this is lead then it's almost certainly cracked. Max length for lead is around 1.5m usually. Thermal expansion and contraction will crack long lengths of this 'thin' lead.
The roof material adjacent to the chimney
Very common for tiles etc. to be broken underneath the metal flashing etc. They never get replaced as the flashing makes this difficult. Heavy handed installation of flashing often leaves tiny cracks in the tiles which form into full splits over time due to weather (hot/cold/frost etc.)


My best guess is the flashing wasn't great to start with and the repairs haven't been effective. Plus the top flaunching needs to be in one piece. 
If it is the flashing you might find that the waterproof sealing of the brickwork has made leaking worse, by the simple fact that the water now runs straight off the brickwork surface and onto the ineffective flashing in greater quantity. Brickwork naturally soaks up a certain amount of water and naturally evaporates the same in dry weather.