Great Western Coffee Shop

From journeycheck:

Cancellations to services between London Paddington and Slough
Due to a fault with the signalling system between London Paddington and Slough some lines are blocked.
Train services running to and from these stations may be cancelled, delayed by up to 60 minutes or revised. Disruption is expected until 13:00 19/08.
Customer Advice
South West Trains, London Underground, Cross Country and Chiltern Railways are conveying passengers via any reasonable route until further notice.
Additional Information
A freight train trying to access Southall Sidings has come to a stand accoss a set of points blocking lines between London Paddington to Slough.

DB Cargo accepted a freight train into their yard at Southall onto a line which was already occupied by another freight train, end result being it didn't fit in the yard and therefore it blocked all 4 main running lines. 

Which way was it going into the yard? From Acton or Hayes?

From Hayes.

I see it should have been held in the West Drayton loop for over an hour but wasn't, possibly hence the confusion.

That'll be the one. I doubt we will find out exactly what happened, it is possible the loop could be out of action (points failure) or something or a simple, but very costly, mistake by the shunter who gave the slot to the signaller, there was a free stabling road at the time.