Hi Sue,
You need to get that tyre changed before you use the caravan - cracks in the side-wall greatly increase the possibility of a blow out.
Did the dealer say why the tyre had cracked? I'm assuming that the dealer will be replacing this under warranty.
And your contract is with the dealer - not with Swift nor the tyre manufacturer - so it will be your dealer replacing the tyres, they will claim their warranty costs from Swift, who will claim from the tyre wholesaler, who will claim from the tyre manufacturer.
Robert