We pulled our fake gas fireplace and almost had a new one installed today. The people delivering it could do the job as the hole for the vent didn't match up to the new fireplace. After a couple of hours trying to make it work, and insisting there wasn't an attachment to get the two parts to fit, the guys left. Our contractor summed it up perfectly when he said "stupid".
The owner of the company came out and said he would fix the problem and install it personally on Thursday. What a relief!