I have an off-peak return Hartford to Derby for tomorrow (Saturday) with Snr Railcard (£9.55).
I have a CCST and it clearly states "Valid only via Stoke on Trent" restriction "B5".
However the journey planner on this site gives me the option to travel via Tamworth for this same return fare, is this valid?
Does B5 only apply Monday-Friday?
B5 is the off-peak time restriction for the ticket, which does indeed only apply Monday-Friday.
However the route restriction of "via Stoke on Trent" applies at all times
As far as I can see, the RailUK journey planner only offers an itinerary travelling via Tamworth with a more expensive ticket or combination of tickets. All of the £9.55 options show travel via Stoke I assume you're looking at the 0704 from Hartford? The National Rail journey planner, if you add "via Stoke on Trent", will also show the same itinerary. Note that although the train you're already on goes to Stafford, you need to change trains at Crewe in order to travel to Stafford via Stoke on Trent. You would therefore be travelling both via Stoke on Trent
and in accordance with an itinerary shown by an official journey planner, which AFAIK would make the journey valid, if not entirely within the spirit of what the route restriction was intended to mean