I read on another forum a couple of years ago that drivers worked 6 on, 4 off, with Sundays in the working week. They could be booked on/off at either Paddington or Old Oak Common, depending on the diagram.
Because of tight turnaround times, the shuttle between Terminal 4 and Terminals 123 is run with two drivers, one at each end of the train. I expect some drivers love the 'not very productive' nature of these turns of duty to be great, and other drivers hate them because they are boring.
Don't take all that as Gospel though, like I said it might be out of date information.