I'm very surprised to see that and also very worried. The thought that I might have to hand over £50 shakes me to the core!
Not legitimate, and also it seems very unlikely to me that this is some form of cunning advert that we have allowed on. I know we've had noisy ones and the odd pop-up slip through the net, but nothing like that.
I'm hoping it's a problem on your machine. Can you read through this and see if it fits: