I have mould appear in the front bedroom in the corner riseing top to bottom, i have a flat roof under the window which is the porch but i have noticed there is water sitting on top ?.

without looking at the problem cannot really say for sure, but it does sound like you have a problem with the roof either the covering has cracked or the flashings have failed. post it in the roofing section of the site and get someone round to check. good luck alex

Find out why the waters pooling on the flat roof, could be a small leak over a few years, caused the underlying boards to get soaked and sagged, causing the pooling.

if it is flat roof they generally from time to time have water sitting on top especially if there is no fall. i would recommend you ask one of the roofers on this site to take a look to ensure that water is not penetrating through the lead flashing above the bay