verified.visa.com seems to be down too, according to websitenotworking .com

Could be that it's not you, Natwest, RBS and Ulster card systems have gone down. Martin Lewis just tweeted about it. Perhaps Nationwide is connected or runs on connected systems?

Call Samsung on 01932 454358 and ask if they have a drop-in centre local to you. We are just up the road from Birmingham and dropped in my wife's Samsung S3 that was completely dead and within 48hrs it was back up and running, ready to collect. All repaired under warranty. Her phone is on Orange/EE contract. They have a 24 month/2 year warranty from Samsung. Samsung helpline said we could alternatively send it in to them (with a 7-10 day turnaround).