Sarah69 - OUCH sore cracked heels are no fun, poor you!
Best thing I have ever used was Boots Intensive Softening Deep Penetrating Formula. Apply thickly put on cotton socks and leave on overnight. Wash feet in the morning, dry thoroughly and apply a thin layer of this cream, repeat on the evening as before.Your feet will be soft and smooth in a couple of days.
Once your heels are back to normal apply a good foot cream, i swear by Body Shop Body Butters, I have a few they are effective and smell divine. Apply twice daily give yourself a foot massage you can learn how to do this from You Tube, do a search. Your tootsies will be lovely and soft and relaxed.
All the best to you x