Oh Hughesie... you know things are twice as heavy in the Metric
system. One Kilogram is over two pounds! A 50 kilogram bag of sand would be over 100 pounds that's way to much to lift. Here a 50 pound bag of sand is only 50 pounds. Convert... it'll be much easier on your back!
Furthermore you have all that weird upside down gravity to fight. Things are much lighter up here because we are lifting right side up. To be easier on your back you might want to think about moving to at least a more equatorial location.

Take care of yourself kid, you only have one back!
Oh as for the back support belt. I heard about a study that showed that people were not wearing them properly and getting injured more than those who didn't use the belt. Apparently people said, "I'm wearing my belt I can lift really big things now!" Then still lifting with their back, and not their legs, they tried to lift something way too heavy. The back support belts are good but only work if worn properly and proper lifting posture is used.