Catherine Schell (the Countess in City of Death) has revealed that she has been interviewed for a feature by the Blu ray team, strongly indicating season 17 will be coming sooner rather than later. So 10, 26 and 17 next year?
I haven't bought 12 (and given it's OOP and riddled with errors probably won't unless it gets a re-release) and am still on the fence with 19, but 10 is very solid and 26 and 17 are two of my favourite seasons ever. I'm almost tempted.
