Are there any regularly updated stats available which show the time in
days taken by the Royal Mail to deliver letters?

Maybe on a week by week basis, so I could look at the stats and say:
"If I order a widget and it is sent by first (or second)
class letter mail right now, then it will probably take
approximately x days."

Of course my question reflects the current Xmas/New Year arrangements as
I am going to need to have parts delivered despite the holiday season.

I would far rather prefer not to have the cost of a courier delivery
especially as it is inconvenient for someone to stay in for the
delivery.