Went down to the stadium at 6pm to see what the crack was with the trains - walked straight up to the windows, by the way - as above, 7.38am departure, four hour ride to Wembley Central (not Stadium, as had originally been stated), £51.50 for season ticket holders.
Ended up coming back home and going for a much less straightforward route that basically involves driving down to Crewe in the morning and getting a slow train from there to London. Probably won't save much in terms of time, but it's only £15 return for the train bit of it, so with two of us going down it'll save a few quid over the official option (which you're supposedly not allowed alcohol on anyway).