Hey Bill,
I have used Heritage, Moss, AH Spares and John Skinner over the past years. Heritage is certainly the best but a close second would be John Skinner or AH Spares. I have always thought Moss cuts their patterns so close to save waste or actual leather/vinyl that you need to be careful when installing. Seat bottoms were troublesome. If you're doing a red interior, most seem to have a more burgundy red than what I understand was more correct for Healey's- a "red" red, not bright red but not burgundy either. I just installed new seats and door panels from John Skinner in my 100M and they have a truer red which to me seemed correct. Since I did just do this you will have pay about $100 for shipping to US from UK but make it up on the purchase price.
Good luck,
Mike