leaky upvc windows external corner joint

[vernerbongo2]

Hi, I have a small square sunlounge and it has leaked around the two external corners (two frames meet at 90 degrees)
The water gets in must run down to the sill dpc and then wets the revails of the plastered walls.
What can I do to seal this? I have used silicone on the outside before but it leaks again after a time.

 

[DAZB]

Fit a length of PVC angle over the corner junction and lay two full beads of silicone on it on both faces, offer it up and then seal the top and edges too. Should keep out the rain for good.