Bob is trying to help, tubby - he doesn't want to be the cause of anyone getting hurt.
Remember the cutter belt has a spring-loaded pulley to keep tension on it. If that spring is not stretched when the clutch is engaged (that is, deck is lowered) then the belt will slip. If the spring doesn't stretch, the belt is worn out or something else is wrong. If you find the belt is worn, you will have to replace it as Bob said.
You should also check the drive from the engine to the clutch. That belt may be worn, or there could be a problem with the clutch due to wear. Since you have at least one part that is slipping, it makes sense to check everything while you are working on the mower. You'll also learn more about it that way, which is useful for the future.
I'll leave this thread open tubby, I hope you'll come back with a report when your back improves and you can work on the mower.
