Posted: 13/06/2013 at 23:44
While tomatoes are largely self-fertilising, there is always a possibility of them cross-pollinating if grown close together. Perhaps you could try growing one plant in a pot placed some distance away from the rest? This year's fruit will come true to type regardless of any cross-pollination - it's the seed and next year's plants which could be hybrids.
I'm not sure I'd call "Peacevine" a heritage variety though. As far as I've been able to find out, it was developed by Dr. Alan Kapuler who is still going strong I believe. Interesting man from what I've read.