It's never going to be as good as a lead flashing, weather or not it would pass building regs I don't know.
I was thinking the original plans Alex . Would have been better, if the roofline was bit lower down the building . Then the roofer, could have fixed the lead into the mortar line , then dressed down , and sealed. Don't think , he could do that , because of the window.
Think the roofer's done the best he could Alex , the only thing I would worry about is , the water coming down the window, usually small hole's on frame to let water out onto cill . Should be fine if their not , covered by the felt .
Teq Ye yea that was my concern they are old upvc windows but he has felted over the sill so hopefully it won't cause a issue