Actually, thinking it through I'm coming to the conclusion that legally the current FIT is 43p, and DECC is on legally dubious ground to tell anyone not to sell at that rate, or to insist that any installation after 12 Dec will receive 21p after April until the appeal at the Supreme Court succeeds or fails.
Whilst DECC have announced an intention to change the FIT, Parliament needs to pass an ammendment to the "Standard Conditions of Electricity Supply Licenses" before any change is legally enacted. Until then the FIT is legally 43p. At present, parliament has not passed any such change.
DECC are planning to put a proposal to parliament changing the FIT for installations after the 3/3 however this has not yet been passed by parliament. Even when this is passed by parliament, legally the FIT for installations before the 3/3 will remain 43p.
DECC are blocked from introducing any change to the license which would introduce any change to the FIT for installations before the 3/3 by the High Court and the Court of Appeal. As such the FIT for those installations will legally remain 43p and legally, they should expect to receive 43p even after 1 April.
If DECC wins the appeal at the Supreme Court then they have stated that they will introduce legislation to retrospectively change the FIT for installations after the 12/12 and the 3/3 and presumably recover any overpayments retrospectively so incurred (there is still a possibility that parliament would reject the change). But DECC's intent to appeal doesn't change the fact that until such retrospective legislation is actually passed, the legal situation as determined by the Electricity Supply Licenses will remain that the FIT for installs before the 3/3 is 43p.
So anyone selling at the 43p rate now is selling at the rate defined in the current legislation as per the "Standard Conditions of Electricity Supply Licenses". It is a little difficult to accuse someone of miss- selling for selling at a rate which is in black and white in the current statute books! If DECC did win the appeal, any claims of miss-selling would be entirely due to the retrospective nature of the amendment that DECC has stated that it would then introduce to the electricity supply license. That would make any case of miss-selling legally very messy to resolve - which is precisely why retrospective legislation is generally seen as a very bad idea, and why four judges so far have refused to allow it.
P.S. the above analysis is from a logician, not a lawyer!