Hi, i actually work for the Royal Mail (Please don't hate me
) Far as i know the vast majority of Deliveries of all types happen perfectly but no service is 100%. Royal Mail have quite a bad rep but this is mostly because they are the only delivery service in the UK that are regulated so they are the only ones that get fined or punished for losing mail so they are the ones people here about in the news.
If the item was sent SD then should arrive next day before 1pm, including saturday. If this doesn't happen then the sender can claim back the cost of sending, and both parties can claim back any money they lost as a result of the failure to deliver on time as long as the address was right, includeing postcode, and was legiable.
To find out exactly what you are entitled to the best course of action is to call the local sorting office and speak to the office manager or (sub-postmaster if a post office)