if it is a dyson dc07 yellow it will not have a clutch which disables the beater head if it is any other dyson 07 you will need to take it to a repairer as it requires a special tool to do a job. It is a bad design really with the clutch model it has been designed so you have to call an engineer to change the belt. If it is an dc07 which is yellow it would be really easy to fix just buy a new belt from currys but it has to be a genuine dyson belt this cmes with some really good instructions which tells you how to change it. I am not really a dyson person i like the old paper bag vacuum my self.
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Try fixfault.com to fix vacuum cleaners.