Hi Jamie,
We bought our property in June paid our solicitors for what we thought was the gas connection and meter and have also received an email on Friday, from Virtus Energia with an invoice for 400 euros which I also think is astronomical. I sent this to the solicitors and they say like yourself it's for the gas contract and the payment I made was for a surveyor to visit the property to check that it was safe to connect gas, which I find hard to believe as previous to my payment there was no gas connection or meter!
I'm confused as thought that the only bills I would now receive are for consumption of gas. Like yourself I am reluctant to pay this as not 100% sure how legitimate this is.