The bean bag has arrived in amazing time: just one day after ordering. But the weather has changed! It's pouring down in B'ham!
The bag itself seems like a great size for leaning against, but too small for an adult to sit on. Hopefully, the weather will change again and I'll be able to tell you the results of actual use.
Hopefully, the further suggestions given will help others looking for comfortable garden reading. The large size of bean back mentioned by pdurrant looks interesting for sitting on.
I deliberately avoided cord, or other material with a pile, because cat fur sticks to it and is hard to remove. Cotton/polyester is much easier in this respect.
I just hope the British summer returns.