[Pony Girl]

Is it too late to plant gladioli corms (not sure if that's the right terminology!) this year?

Also can they be left in the ground over winter?

[Maarten]

It is still fine to plant them. They will just flower a bit later than if they had been in the ground already, although they may catch up a fair bit. They are fine to go through the winter, especially in the south near the sea.
Maarten

[sunshineband]

In fact if you stagger their planting, you get a good succession of flowers.

The only problem with them staying in the ground over winter is the huge number of tiny offshoots that then grow all around the big corm: they come up like blades of grass 

[miggs]

One of the best flowers you can have as far as I am concerned. Brilliant as a cut flower. Agree with sunshineband 100% about leaving them in over winter, best to get them up if you can.