Since when do BG service gas fires?
If this is a class-1 brick chimney...
- Repoint the chimney as necessary
- Close to the fire it fire cement is generally used
If this is a precast flue...
- Generally further inspection is needed
- Is a precast block miss-aligned or cracked
- Is mortar missing, dry joint etc
If this is anything else further investigation not "sticking plaster".
A lining for a gas fire is not expensive, it's just a plate top & bottom, plus a pipe. I think ?Wonderflue? is one type. A lining for a wood burner conversely is very expensive re double wall and so on.
You are allowed a certain limited degree of leakage from a chimney BTW, however the precise details matter. It would be worth checking with a Gas Safe person and posting to uk.d- i-y where some exist.