You could try cutting the bottom out of a large pot, then cut out the holes in a growbag and stand the pot on top of the growbag, half fill the pot with good compost, put your plant in this, then fill the pot with the compost, tomatoes are stem rooters, so the stem that you covered up will develop roots, and the roots at the bottom will grow into the growbag, giving you more root growth, then just feed as normal.
If you have soil borders in your greenhouse, you can use the same method, only without the growbag.

thanks will try that,i have noticed even though we use fairly large pots when all is finished the are really full to brimming with roots.thanks again.