Train crew rosters are should be published to traincrew 5 days in advance.  Once they have been published that's it, they can't be changed, unless the driver or TM^‡(resolve) agrees.  All the company can do is change the content of the diagram within the published booking on and off times.
Traincrew also get 5 days notice as to whether their leave has been granted.  If a driver is available for rest day working and hasn't been booked out they can be asked at anytime whether they want to work.

RMT^▸ (National Union of Rail, Maritime & Transport Workers) will be rubbing their palms with glee. A perfect opportunity to call strikes.

How about the railways sort out Sunday rostering before attempting to find staff to run overnight services.

All the lines through Bristol Parkway will become fully reversible (including the Up Passenger Loop and Down Goods Loop).  This will happen during the F4T commissioning later this year (2018).  All these lines will be accessible from the London, Filton Abbey Wood, Patchway and Avonmouth directions.