I'm registered and would not recommend any DIY on gas, the regs cover the installation, the flue and the entire gas installation.
I oversaw a contract of replacing 1000 units in Solihull and I've seen enough for a lifetime, they had asbestos flues and had to be properly removed, get professional advise.
Regulations change regarding ventilation etc so a straight swap may require additional work to comply, Also all new gas appliances have to be registered to the local Council by law and certificate required, if you do not get this it will not be included in your House Buyers Pack and you may not be able to sell your house.
CORGI is the governing body and they have just lost the job, new company to oversee the work in next few years so all the CORGI badges on vans and letterheads useless soon.
Gas fitters have to registered separately for each item of work; eg
Cookers, servicing, boilers, wall heaters, lpg.
Each of these requires a separate exam, and pass, usually 3-5 days off work and £350 - £900 each plus loss of earnings every five years to keep your qualifications up so just because a fitter is CORGI registered do not think he is automatically able to legally fit your warm air unit.
Yes thats right, get qualified become a doctor, prescribe medicines until you retire, no problem.
Want to fit a cooker? back to college for new exam every five years until you pack it up, that's why many old guys no longer work with gas.
Do you know any other trade that has to re qualify every 5 years,
It can get complicated, get a professional, or save a few bob and kill your family, it's gas for Gods sake.