after having read the comments, this is an interesting situation however if you look at the gas industry unsafe situation procedure booklet.. part2 section 9.3 (general). this is not full quotation but is a summary it is saying,
" some old appliances exist which do not conform to current standards and as such need to be assessed individualy, as standards have changed due to research and incidents investigation. it is the gas operative who makes the idividual assesment. if an existing installation is not in accordance with current regs etc it should be considered to be NCS, providing it can only be used without consitiuting a danger to anyone and is not likley to do so."
Now could this be considered safe or is there a possibility of the pilot going out and causing a gas escape that is the fundamental question. If there is a possibility then by sub clause 6.4 you may upgrade to an AR. Again this is down to the individual who is after all the person signing to say it is or is not safe.
After all if this went up in a fire ball would the HSE look at this reg and ask the question do you consider that the system was safe in light of the recent explosion? all you gas engineers who have been doing this for a number of years I am sure have seen some real poor jobs and as such has this engineer not endevoured to ensure this customer was safe? did he leave the premises safer or more dangerous? Is saftey the main aim as a gas engineer. As for HOBs without FSD would you put your hob gas on and go out to work or on holiday? and that is why they are a different prospect.
As you stated you are thinking of changing this gas fire, my advice would be to follow that line then when you go out you can rest assured that your children/family are safe!