First I noticed the yellow streak on the screen and contacted Microsoft. They offered a replacement Surface Pro if I paid £568.
Then I noticed that the screen was bulging away from the case. I sent a photo to prove this to Microsoft support. They said not to use the machine immediately and that they would replace it free of charge as it was just less than 3 years since I got the machine. I would point out that this machine was supplied by Microsoft when the original one had flickering screen and hot battery issue. So it was out of warranty but less than 3 years old
I was asked to upload photos and proof of purchase. When not a lot happened I made further phone calls but eventually they sent me another Surface Pro 4.
As this would be my 3rd, I decided to cut my losses and sell this brand new - or at least refurbished - Surface Pro 4 with all its accessories on eBay. Had they sent me a Surface Pro 5, I would have kept it.
So moral of the story is if your machine is less than 3 years old and you have a yellow streak on the screen have a look to see if the screen is perfectly flat. If it is bulging, do not carry on charging and contact Microsoft to get a replacement