I replaced the fuse again. I am going to leave the PSU plugged in for a while to see what happens. This is because the last time I replaced the fuse I did plug it into the C128 which caused a load hum through the monitor speakers. However, the fuse blew after I had unplugged it from the C128 and while I was testing it with my MM.
Anyway, I get 5.21v DC when testing pins 1 and 4. However, I only get 2.54v AC when testing pins 3 and 5.
The troubleshooting continues.