> ... because I was told once that if you send someone a legal notice in Britain by Royal Mail, saying, e.g. 'You have 28 days to comply blah blah' the key date

> ... because I was told once that if you send someone a legal notice in Britain by Royal Mail, saying, e.g. 'You have 28 days to comply blah blah' the key date is the date it was posted, not the date it was received, unless the recipient can prove they received it late, not sure if that is still true since the post office was privatised. It is. The Interpretation Act 1978 section 7 is where this bit of law currently resides but the principle is older than that: Where an Act authorises or requires any document to be served by post (whether the expression “serve” or the expression “give” or “send” or any other expression is used) then, unless the contrary intention appears, the service is deemed to be effected by properly addressing, pre-paying and posting a letter containing the document and, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.