Check whether your agreement is a conditional sale or a credit sale agreement. If it is conditional sale then under the VT clauses in the CCA (1974) if 50% of the total amount due has been paid you can send it back with no penalty. You should have an option to purchase clause for the balloon payment under a PCP agreement (i.e. to acquire the vehicle if you want to) however check your agreement as this is usually optional. In other words the total amount due should not really include the "option to purchase" payment in case they try and include this.

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