You would need a Gas Safe Regietered engineer (Previously 'Corgi Registered) to undertake at least the refitting of connections to oven , and hob and fridge (if in the same location). ....
I'm sorry Rob but there a number significant in accuracies in what you have written. Having previously been Corgi Registered is no guarantee that a person or company would retain Gas Safe registration when they completely took over the process in 2010. These days reference to Corgi is has no relevance.
The Gas Safety (installation and Use) Regulations require all work on gas systems supplied by pipe must be carried out by a competent person who is Gas Safe Registered. This includes installation, connection or disconnection or decommissioning of the system its appliance's.
It is a duty on the fitter to ensure all work is carried out to the relevant standards and codes of practice and must include the required testing of the system.
Failure to comply is a criminal act.
Mobile Homes or static caravans are treaded as full domestic dwellings and are not exempted from the GS(I&U)R
However, the The GS(I&U)R also include certain specific exceptions where the regulations do not apply to the private owner of either a Self propelled(Motorhomes) or Touring caravans where gas is supplied from a cylinder used solely by the private owner, and not loaned or hired for use by other people.
I personally think this exemption is a ludicrous abdication of the law, having seen what can happen when non professionals have interfered with gas appliance's and seriously poisoned and nearly killed family members with CO as a result of tinkering with flue pipes and gas feeds.
Whilst the GS(I&U)R may exempt personal tinkering in private motorhomes and touring caravans, there is no similar exclusion to the requirement that all works must be carried out in accordance with relevant standards for quality and safety testing. This does allow qualifications other than Gas Safe registration to work on LPG based mobile systems
The HSE could still prosecute if substandard gas work has been performed in motorhome or touring caravans by the owner.
Other pitfalls are if a caravan is loaned to or used by someone other than the owner, the act may be construed as £0 Hire, and thus reintroduces the full force of the GS(I&U)R.