3 x 2m flat roof. Damp in corner of kitchen extension

[Grumlethegoat]

Hello,
I have a small extension on the kitchen at the back of the house (3m x 2m) and it's causing damp in the corner of the kitchen wall.
I noticed a musty smell in the kitchen last year after we moved in and took one of the kitchen cabinets down and it was thick with mould so I stripped the area of wallpaper and applied a mould remover/killer, used a Flexacryl type roof repair on the roof and then after a month or so things seemed ok so I re-decorated, but I have just noticed damp showing through on the wallpaper again.
I really can't afford a new roof.
Should I re-do the Flexacryl?Any other ideas would be great.
Thank you.

 

[Burnerman]

Pics of the damp, please! Although it does sound like the damp is coming from where the extension joins onto the original part of the house, so there could be a flashing problem.
John