I would start by engaging your local building control which forms part of the district council.
They are responsible for ensuring all building meet HSE standards, and also set the requirements which the boiler fitters have to work to.
Your neighbours have probably had a boiler service, which has resulted in the additional 2M pipe being added as part of those recommendations, which means they probably meet the Gas Safety requirements outlined by building control.
Though you may not get a prompt response from Building Control (they normally take weeks to get back to you - each council is different), they will attend, at no cost to you.
Should they find the installation wanting they will issue a notice for the property owner to make suitable modification to address the areas of concern, then come back and reinspect before signing it off.
However if they feel everything is by the book (which might be what the 2M addition achieves), they are unlikely to make comment, or be of any further assistance to you.