In the interests of safety to all and property – you need to call in the services of a suitably qualified gasfitter to shorten the pipework and adapt it to the logset you have purchased.
Unless you are qualified in working with gas appliances do not attempt this kind of work yourself as a leak left undetected can produce very dangerous and explosive results!!!!!!!!
Anyways an unvented log set should be fitted by a gasfitter to ensure there is sufficient air for both combustion and ventalation purposes.
Hire a professional a small cost now will ensure a safe and happy customer.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians