Thanks, that's a great help, I have guessed then that the (unbalanced) reaction is:
K2Cr2O7 + Fe(NH4)2(SO4)2 + 7H2SO4 → K2SO4 + (NH4)2SO4 + Cr2(SO4)3 + Fe2(SO4)3 + 7H2O
But I have been fiddling around with it pretty much since your post and I can't balance it (feeble brain huh). I can manage to get the ammoniums andand everything else to balance, but never the sulphates. I couldn't seem to get enough of them on the left hand side, so I changed the H+ to H2SO4, but even so, I couldn't balance it..does it involve some pretty large numbers?