Hi Adrian, two separate issues here in my opinion, on the warranty, and the purchasing of additional Son's Mark XV:
a) if you bought from an authorized dealer, then your watch set should come with full international warranty. The guarantee card may be 'unwritten', but still be valid - is the Authorized Dealer's stamp on it? ( it may not always have a signature), are the model reference number, and the casing serial number, imprinted on it, or written on it (I have seen both instances) ?
b) on whether you can buy more than one Son's Mark XV, if that dealer is not now geographically accessible (being too far away), I don't think it is logistically or economically feasible to order more Mark XVs, so it renders how correct their policy of not selling more Son's watch moot (please feel free to correct me). But if you do look around you online may find some dealers selling the Son's Mark XV (since this is very new - perhaps some time in the near future this could be sold), but as this forum is non-commercial, 'what to buy' posts and external commercial links are not allowed.
@ Alan, I think the additional Son's Mark XV, indeed the set, need not be engraved -- all up to the buyer.