Shaun, I have the same problem. On 7 April, I was able to speak to a very nice fellow at OS Customer Service who admitted that he was having problems with the new site also. Then I tried again about a week later and the result was an email from a less helpful customer service rep stating that the issue had been fixed and that the problem was my fault. He suggested that I update my browser to the latest version, clear the cache, and start over. Well, that is NOT the problem. I have tried three different browsers, Safari, Chrome, and Firefox, all of which are the latest versions. I also cleared the cache on all of them. Guess what? The system will not "Generate PDF" when you click on that button. It simply times out. However, I came up with a work-around that you might try if you have a large computer screen. I found that when I hit "print", set the scale to 1:50K and A4 paper, when the map print area shows, instead of clicking on the useless "Generate PDF", I take a screenshot of the map area. I then convert that to a pdf and print it out to fit an A4 paper, and it comes out perfectly. BUT I think your screenshot must be at least the size of A4 paper. You might try that.

No help here, but I've had nothing but trouble with the OS maps (desktop and mobile app) this year. Spent lots of quality time with customer service, too. For printing, I ended up taking screen shots and pasting them into Google docs for printing. Worked pretty well actually. Good luck!