They stack, in that order. Your free trial runs first at no charge, so you don't “burn” any of the discount during the trial. Once the trial ends and your first paid billing period begins, the discount kicks in for however many periods it covers (our launch offer is 50% off your first 3 months on monthly, or 50% off your first year on annual). After that, your subscription continues at the standard rate. You won't see the discount on the £0 trial line, but it's locked in and will show up on your first paid invoice.