From the sounds of it it's the calf rather than the feet part of the boots you want stretching.
You can try wearing them until the leather stretches, if it's thin leather it may work, the only problem is you're putting a strain on the zip.
The other option is to have them widened at an independent shoe repair shop. This is done by replacing the zip with a 'wider' one, giving you more leg room. This can be a little expensive tho, from about £20 - £25 per pair.
Good luck stretching them!