Aftermarket warranties, as with manufacturer warranties, are defined by their terms and conditions and what is, and is not included, so we would start by reading the T&Cs in detail to determine what they say about these suspension components. Bushes and ball joints are possibly considered as 'consumable' in that they eventually will wear out and need to be replaced, but at the same time it could be a reasonable expectation that they would last longer than they have. However, if they are considered consumable then it may be that the T&Cs will exclude them regardless of timeframe. If they refuse to accept responsibility, you have the option to get quotes for the work to be carried out and send them to the provider, stating that if they continue to refuse to pay for the work you will claim back the cost from them. You can also check if the warranty provider is a member of a trade association such as the Motor Ombudsman and take your complaint to them.