They should be cancelling both sets of tickets and re-issuing.
You should not have to pay a single penny for their incompetence.
My thoughts too. Surely they should have the ability to void them and just send me new ones?
Any sensible retailer who does a lot of mail order business would have a Freepost address for returns. It might not be publicised on the website, but it would be available for this sort of transaction.
The address on the back is for The Fleming Centre, Crawley, Sussex. I'm guessing this is the single source of tickets by post? I've had tickets from many
TOCs▸ sent from this address.
I'm hoping to follow this up further tomorrow. Giving them the weekend.