Virgin are saying available until the 5th June.
London Midland are available until the 16th May with this on the website:
"We are not currently open beyond these dates due to finalising the setup of the May 2015 timetable. We shall be opening further weeks before 28th March 2015."
XC & EMT have released their advances for that date, so it's probably a case of waiting until Virgin WC & Lon Midland have released their advances for that weekend.