Glow-worm Capricorn 240/6 Gas Liner Terminal diameter?

[Tony59]

The Gas Liner Terminal is missing at the top of my chimney. I have been advised to get one installed. Whilst shopping around I read that it is essential that I get the correct diameter fitting. I see that they come diameters between 4" and 9". Does anyone know which size it is likely to be?
It would be good to be able to purchase and install it without having to measure the existing liner first.

 

[namsag]

Its a FIVE inch gc1 terminal you require

 

[Tony59]

I am arranging to have the Gas Liner Terminal installed. Someone has recommended that I take the precaution of checking if the chimney is blocked with a birds nest or something else first. Can anyone recommend a way to check that the chimney is clear?
Thanks