The answer is there in the question itself....if the initial sale price includes price for warranty also then you have already paid VAT on it. So no additional VAT will apply unless additional consideration is charged for warranty replacement.

In case warranty is purchased, then the supplier should not charge you the cost of spare part itself and thus no VAT should apply. If the supplier is charging the cost of spare parts, then you need to revisit the warranty terms and check the warranty inclusions. In such case, Vat might be charged by the supplier on the spare part charges..