I've complained today to Royal Mail and have also emailed Helen Hayes, as others have done. The post situation really is dire! Am waiting on couple of urgent things and haven't had post - or seen a postman - for about 2 weeks!!! :-(
This happened to me and I went to customer service desk with receipt and copy of the fine and they were able to get it cancelled - definitely give it a try - the Sainsbury staff were very helpful - and apparently the time limit used to be 3hrs but they changed it last year. Good luck :-)