Hello again,Henry!
Motts are based in Aylesbury, but they also have pick-up points in Amersham, Aston Clinton, Bletchley, Chesham, Haddenham, Hazlemere, High Wycombe, Milton Keynes, Princes Risborough and Wendover:
http://www.mottstravel.com/DesktopDefault.aspx ?ptabindex=2&ptabid=145
Koncept Travel (in Milton Keynes) haven't updated their website for 2 years but it might be worth phoning them to see if they still do day trips:
01296 714382
Your local tourist information office will probably know of other firms running day trips. Why not give them a ring?
Milton Keynes: 01908 677010
Aylesbury: 01296 330559
Marlow: 01628 483597
High Wycombe: 01494 421892
Wendover: 01296 696759
Buckingham: 01280 823020
Chris