When I started full time at Royal Mail, I had to type in some sentences with some rubber put over the keys, so I couldn't see what key I was pressing. The only keys used are the middle keys on the keyboard, plus the "E" and "I"
You had to have a minimum accuracy but I was never told what it was. I ended up making a few mistakes and scored about 93%
One trainee was called away, and then after about a minute wait, we were all told we passed (without the other person present)
It is not expected that you have to finish the paragraph, as they're more concentrated on accuracy.
However, this was almost 3 years ago, so things may have moved on, and if you're going through an agency, it might be only Christmas work, and they do a different test altogether which I imagine is far easier as you can't weed out too many people when you need over 1000 temporary staff.