Deafnat80,
If you've kept the receipt slip the post office gave you on sending the letter recorded delivery it has a serial number on it and both the post office website and telephone number. You could put your mind at rest by logging in or ringing then inputting your reference number on the receipt you were given. It will then confirm the date and time the recorded letter arrived. Note it just in case.
Furthermore you have kept the copies of the cancellation e mail(s) ? I know I'm not there and can't feel what you're feeling but try not to worry you have done the correct action. Tip for the future! The local council offices or library may have a photocopier and will make you a copy for a few pence well worth it!
Meanwhile Huckster information is sound
Regards,
Paul