Basically, D Fare Basis exist on all Short-Haul routes, with a few exceptions (maily where BA is the only carrier - in which case there is effectively no difference between a C and D Fare, so BA only offers a C Fare Basis).
The easiest way to check availability is either by phoning BA or using the advanced user version of ITN.
Paul, regarding your comment about a C Class cabin on LHR-AMS, there may not be much difference - but if you want a fully-flex ticket you may as well buy a C Fare basis, the different between this and M class is peanuts (maybe 10 - 15 quid). It is worth it just for the double miles and points.
As I have said many times, there is no IATA C FARE BASIS in Europe - you buy a full fare ecomony ticket and you are given C CLASS OF SERVCIE.
Nick
[This message has been edited by Merry (edited 05-06-2001).]