Firstly, Cran
well is nowhere near Cran
field!!
If you can catch the 0857 train from Nottingham to Sleaford, you should arrive at Sleaford at 1003. It's only about 6 miles by road from the station to Cranwell, so a taxi may be your best bet.
But you could well be quite travel weary after the longish journey - so it might be better to get there the previous night?(the Carre Arms is very the railway station in Sleaford. No idea what it's like these days, but have a look at
http://www.carrearmshotel.co.uk/).
And stay off the pop if you go the previous night! Good luck to you.