If they posted it this morning then sometimes the post is collected at lunchtime and it gets on its way quicker, so it should arrive to you tomorrow, but if it's posted later on in the afternoon then sometimes it doesn't always guarantee to be next day delivery. In the good old days first class post meant it would get to you by the next day but nowadays it seems it's not always a definite!
A true postcode lottery! It depends on where you post it from and where it's traveling through and going to. If Oxford is involved anywhere in the equation, chances are there will be a strike on!! ;o)
as a postman, be aware in some areas the last collection from some boxes is 6am or a little bit later. Yes, 6 am, so you can post in the afternoon, but it won't be collected until the next day.
Also be aware - for those of you keen to blame the Royal Mail - that a lot of what has gone wrong is a direct result of Postcomms influence and the deregulation of the post service. If they had left it alone then things would be a lot better, for you and us postmen.