I know the regulator is very strict on the time scale for 1st Class arriving the next working day, in the high nineties I believe and Royal Mail's own website says 1st class is a next day service all be it they aim for that but it mentions nothing about 4 days....I think some muppet with no clue has just fobbed her off to be honest.
Now the letter in question had the correct sized stamp, the address was clearly printed as well so no excuses there and most importantly I took it to work with me on the Saturday and it went out with the dispatch to the mail centre just after lunch time so it definitely went and wasn't sitting in a post box until Monday afternoon. It should have arrived Monday I believe and at worst Tuesday.
Personally I think it's sat in a cut off bag until Wednesday or the duty it's on has been lapsed