If you live in a property (most likely a flat or apartment) which has a room sealed, fan assisted boiler with a flue which is hidden behind a ceiling or wall, then a Gas Safe registered engineer will be unable to inspect the appliance.
As the flue removes fumes from the boiler, it needs to be accessible for inspection. A blocked flue, or one in poor condition, could put you at risk of carbon monoxide poisoning (although being unable to access the flue doesn’t automatically mean that it is unsafe).
If this applies to a property you own or live in, then it’s a good idea to have inspection hatches fitted to access the flue. If a Gas Safe registered engineer visits and is unable to view the flue they will classify the appliance as 'At Risk', which means: