Chiltern Advance fares

[calc7]

Is there any reason why Saturday Chiltern advance fares are not available past next weekend (http://www.nationalrail.co.uk/times_fares/booking_horizons.html), and indeed past early September on Sundays?!
I'm looking to do Birmingham to Marylebone on 21st January...

 

[barrykas]

Looking at the Chiltern Changes to Train Times page, details of Engineering Works beyond the end of December have yet to be announced.

 

[calc7]

Looking at the Chiltern Changes to Train Times page, details of Engineering Works beyond the end of December have yet to be announced.

Thanks - I didn't think of checking there. There was a thread a couple of months ago speculating about Chiltern fares and their intention to get everybody onto the £25 SSR/£50 SVR/£75 SOR rte High Wycombe tickets, so I assumed the worst.