(EDIT: as per the Comments, it seems that there is a technical issue with card numbers whose 5th number is a 0 resulting in a “by invitation only” message and a refusal to register you)
To qualify for the £100 rebate, you need to spend £300 or more on pre-paid hotels, plane tickets, car hire or experiences in one transaction before 26th February.
You MUST also register your Platinum card at this website before booking. Only 6,600 registrations will be allowed.
This bonus is in addition to the standard bonus of 1 extra Membership Rewards point per £1 spent, on top of your base earnings.
This offer is probably best used for airline tickets or car hire. Bookings hotels through American Express Travel runs the risk of it being treated as a third party booking and not qualifying for points or status credit – although you have a better chance of getting credit via Amex (because of their huge business travel booking business) than Expedia etc. That said, for a £100 discount you may be willing to take the risk.
Note that hotel bookings via Fine Hotels & Resorts are specifically excluded, although this would be the case anyway as these bookings are never pre-paid.
If there is a chance that you might be spending £300 on flights or hotels before 26th February, I recommend that you register here just in case the 6,600 target registrations is hit. Don’t bother trying if you don’t have a Platinum card as it will reject the registration.
PS. The article I wrote for the American Express Travel blog is still there, take a look if you missed it the first time.