I finally managed W5R3's 20 minutes! It has taken me nearly 3 weeks to get there but I've done it. I really don't know how anybody goes from 2 lots of 8 minutes straight to 20 minutes non-stop!
Rather than just repeat all of the week 3 runs I decided to keep trying W5R3 increasing the length of my first run each time before I stopped for a walk. Then I would maybe walk for 2 minutes and then carry on running. I actually ended up running for more than 20 minutes in total so I knew I would eventually make it. I finally achieved the 20 minutes non-stop after 5 extra runs.
I now feel amaaaaazing!