Water leaking in through upstairs upvc window frame

sounds like the frame sealant has perished and needs re sealed

It sounds like you may have a broken roof tile. What happens is the water gets in through a damaged tile. It then runs under your tiles on your under felt membrane. Which in theory drops into your gutter. But you tend to find the exposed part of the under felt rots. This allows the water to enter the cavity between your bricks. The water runs inside your cavity until it hits your window. That is why the water is actually at your window. This is an easy fix. Just have the roof looked at and I think you will find a broken or cracked tile. And simply replace. You may also consider fitting felt support trays. These plastic trays go below the exposed edge of your felt and support it. They ensure any future undetected leaks run into the gutter and not the cavity. Hope this helps. It should not be expensive job to have done.