Monsoon, I can understand that the anticipated charge may well be both expensive and irritating, but I don't get why it's coming as a surprise, or why you think that it's in some way unfair and/or unreasonable.
If you don't think that then I apologise in advance or your reply, but you're requesting a service from a business that has a goal of being profitable. It seems to me to be an entirely successful way of making a profit to charge a realistic amount, i.e. that reflects the work involved, for reconnection according to the current safety regulations.
It's normal, as part of conveyancing, to ask the vendor about the live services. If your solicitor asked the question then he's discharged his responsiblity, and if the vendor indicated that the gas wasn't live, then he's discharged his responsiblity. If either or both of them didn't, then you may have a reasonable chance of holding them liable for either negligence or misrepresentation, respectively.
If it was the case that you simply failed to notice the implication of a dead gas supply, and you didn't determine out the cost of reinstatement before exchanging contracts, then it doesn't seem fair of you to point the finger at anyone else.