Interesting about the mixte frames. I know my frame is made of High Tensile Carbon Steel which is lighter than normal steel so the mixte style would make a very strong, very stable frame. It's certainly a very good quality bike and was probably quite expensive when it was first bought new.
You can tell I love it LOL
Just had a look at the Raleigh catalogue. I had a Raleigh "Rose" which was similar to the Twenty except it didn't fold. It had Sturmey Archer 3 speed gears (which didn't work) and the thing was very, very heavy. I didn't use it much and left it to the next owners when we moved house.