I'd nip into your local delivery office (Royal Mail) if possible; sometimes the tracking doesn't update and they keep it at the depot if the first delivery was unsuccessful. They should leave a note, of course, but on the off-chance they didn't it will be depressing if they send it all the way back to the sender.
I have had things sit in one of those huge hubs for a long time before and it could be released any day, but alternatively it could just be lost. Since you have a tracking number you should be able to get some advice if you call Royal Mail or Parcel Force (they'll be able to tell you which one it went with if you don't know; if the online tracking mentions a service like Express 24 or Express 48 it will have been assigned to Parcel Force). One parcel I had to rescue for my office was showing as 'retention' for a week before I called up and miraculously someone managed to find out what was going on and I did eventually receive the item. In that case it was due to some hiccup processing the information for figuring out the customs duties required - perhaps they only have one calculator on site and they had to wait several days to borrow it...
R
