Flat roof over bay window leaking - advice please

My 1st flor bedroom has a bay window with a piece of flat roof about 1mt x 3 mts above it which I can access from a window in the attic room. That flat roof has a channel running the length of it to take rain water to the down pipe. It's been leaking constantly for a few months and I've had two roofers round. I don't know what the first one did - but it didn't work. The second one did a bit of raking out and applied Evercryl (I think that's what it was called). It still leaks after the second roofer, but only in very heavy rain.
I' climbed out there to look for myself and poured a few bucket of water on. No leak, which makes me think the bottom of the gulley is not the source of the leak. But I see that the water in ponding (about half an inch deep) right in the centre, which is where I have the leak in the bedroom. So - I'm wondering if in especially hevy rain the ponding gets so deep that it overflows and somehow gets in to the celing below?
If so - how can I stop that ponding? I was wondering if a half-inch layer of pea shingle in that 1 meter length of the gulley would stop it. Or maybe build up a thick layer of mastic??
Any suggestions or advice?

Thanks in advance

Roger

 

It's a Victorian house. I'm not sure what the roof material iss but it feels as though it's a wooden frame covered with felt and with a coating (or perhaps several coatings) of mastic or some such product.

Thanks