repairs to cracked felt on a flat roof

[numpty with a crowbar]

Got a small problem on a flat roof at the front of the house. The felt is generally ok, but close to the edge there is a recess in the roof that forms a gutter, the felt has cracked in the U (on the house side of the U) - drawn a diagram to illustrate

Now I could get a roofer to redo the whole thing (it's probably about 9 square meters so estimate I'd be paying about 250 quid based on what I've read here on per meter prices..... But I was considering trying to patch it by effectively laying a strip of new felt across the channel as per this diagram

Is this a good idea since it's a patch? If I can do it should I just use a single layer of felt? And what should I use - adhesive or proper bitumen? Also how clean will I need to get the existing layer of felt or should I remove it?

 

[big-all]

assuming your roof is at least 15 years old you only need a temporary repair as the whole roof will probably need doing in the next few years

you realy need to peel of the top layer and overlap by about 6 inches towards the house this in turn will give up in a few years but at least the water wont pool in the roof because of the 12mm step

but wait for other opinions as i dont do roofs thats my nebours job