Stopped at Strensham services yesterday for a coffee (like you do) and found they had a new offer in the resturant.
You buy a coffee in a specialy branded mug for £2.15 and you get FREE UNLIMITED REFILLS!!!
Not sure if this applies to all Roadchef services but might be worth checking out Clacket Lane on your way back one trip.
At least it would be one reason to stop there