Ground, pots or bags??

Does this mean that yours are not presently in pots, or that the person says they should be grown to full size in pots. 

Since they are plants I would have thought that the ground is the natural place for them. Mine go in the ground, grow well , flower fruit. None have rotted so far and I am not serious on it all. So the ground works for me, well for my toms.

unless they are in pots they can rot?

That in itself is a load of rot!

I've often grown toms in the greenhouse soil and I'm doing so again this year. You have to be sure it's fresh - which is no problem in your case. I've dug in the contents of my compost bins & thrown in some chicken manure pellets & they seem quite happy.

How ickle are they Mick???     If they are still very small, I'd leave them in the pots a while longer to get a bit taller  or re-pot into a larger pot for a bit.  But like DD I do grow my toms in the borders of my greenhouse, but they say to watch out if you do it every year as diseases and pests can build up, so rather than take out the soil each year, like DD I put in loads off chicken manure, spent grow bags old potting compost plus growmore and stuff, and after 3 years I've never had any trouble.

In my small greenhouses I have plastic and bark on the ground, so I grow in pots in those ones, but they do dry out much quicker than the tomatoes in the ground!

Mine go into final positions at 6" plus.