Plant them in the spring and in the autumn dig them up, wash and dry them having separated the corms (new ones are growing on the top) before putting them in a cool,dry,dark place over winter. I read recently that here in the South East its becoming practice to leave them in the ground over winter.

if they are buried 6" deep or more they will be fine. all mine came through last winter. The cormlets can be planted on in pots though they wont get to flowering size for a good few years yet. I tend to leave the with the adult ones.

If yours are not as deep as this then you can add a mulch to them to help them through.

It is a 'depends' type of answer. If your soil is well drained and you are in an area which does not get deeply penetrating frosts then you can leave them in the ground. Otherwise dig them up. To be honest I have never bothered, it is less trouble to buy new ones in the sales in Spring.
As to the little corms, then two choices. Keep them dry over winter, plant them out in Spring in the Veg garden and let them grow on until they reach flowering size in a couple of years. Or, throw them away.
The purple ones which survive are most likely G. byzantinum (or is it carinatum now?), They are hardier than the South African ones.

Thanks everyone. I've decided to go for the mulch and keep fingers crossed!
Penny