Whilst the letter does seem harsh it also did what it intended which was make you pay. And whilst for them to threaten debt recovery after 14 days of non payment is extreme it's aldo standard business practise. TBH if it's a case of your credit card expiring then if you had been charged the £15.00 once you spoke to them then they would have gotten rid of the fee as it's wasn't your fault. All in all the letter whilst being aggressive is also fairly industry standard i'm afraid.