Parcel cleared customs but still stuck at Heathrow?

I ordered a book from Amazon US as it isn't available in the UK yet. Anyway, so far the package has smoothly got to the UK and it cleared customs within a few hours. Since clearing customs checks though it has been stuck at Heathrow for almost 3 days now with no update, okay Sunday is an excuse, but Friday evening, Saturday and today. I just can't understand how it has made no progress whatsoever.

My order is being delivered by UPS by the way.

Why hasn't it done anything for 3 days since clearing customs?

2016-02-15T01:08:56Z

I think you'll find postal services operate on Saturdays and even if the parcel isn't being delivered you still typically get tracking information on where it is. Most postal services I have used have given me updates at 3am saying it is at some particular sorting hub. This parcel has not had an update in 3 days and I again point out, postal services still operate on a Saturday.

Tavy2016-02-15T01:03:58Z

Saturday and Sunday are not Business days. Its only 9am in the Uk where I am reading this. Did you expect it to be delivered after midnight last night.?