Concrete Roof

[darkknight]

I have a mid-terrace concrete flat roof house with the drainage channel built into the eaves.

The centre of the roof appears to be lead, but the 'gutter' around it is asphalt sheets.
The gutter runs around all 4 houses.
The issue I am having is that water is running under the asphalt. I have sealed it as much as possible but I think it's coming from the neighbours.
This is only showing as an issue in one room as the external wall for that room sticks out further than the rest of the house, meaning the cavity is directly under the drainage channel.

I have had a few roofers out, all of which have disappeared never to be seen again.

The last roofer sounded promising, he suggested we pull back the asphalt, seal the concrete and reattach the asphalt.
Unfortunately he has also disappeared.

I have a scaffold tower and a MEWP so I'm probably going to attempt this myself.

Searching around I found the Resincoat 'Roof sealer and waterproof coating' product. Can anyone recommend it or recommend an alternative that would do a better job?

 

[freddiemercurystwin]

Got some photos, I'm struggling to understand the set-up.

 

[Alastairreid]

If its coming from your neighbours there is not a lot you can do without getting your neighbours co-operation.