Roof Problem?

Hi,

I'm after some advice if possible.

I live in a detached bungalow and I'm having a problem with the front elevation as I'm getting damp in the reveals of my bay window and another window on the front bedroom.

It was quite bad to the point a few years ago I took the plaster off as it was wet and replastered, however the problem has come back.

It's 100% not the case water is getting though the top or the sides of the window frames as the are well sealed.

I did have windows sealed again quite recently as I thought that may have been the problem (they were still under guarantee so the guy that done them didn't mind) anyway I showed him the problem, the damp plaster on the reveals, and even he said there was no way water was getting in through the windows, again he wasn't fobbing me off as I could see they were perfectly sealed, however he thought it could have been caused by driving rain getting through the slates and the water was running down on the top of the window frame and then running down the sides, which I think is quite plausible as there's no fabric underneath them and the other sides are fine as I had a loft conversion done several years ago so the other 3 sides of the roof were off so this front elevation was the only side of the house untouched with regards to the roof.

Is this a possibility, what do you guys think, could this be the cause?

He said the problem could be solved by installing some of these plastic eaves trays?

Would this work, and if so how would I install them as it's a slate roof so I don't think I could slide them under the slates as the nails would stop this, I am right in saying this?

I have Googled this and most opinions say to remove a few rows of slates to install them, however how hard is this to do?

It's really the last thing a want to do but I would if this sorts things, however the joys of living in a bungalow means that they are easy to get to.

Any info or advice would be appreciated.

Thanks