Connecting gas fire

[steq184]

I am in the middle of building a false chimney breast for my flueless coal fire, have already put up the fire resistant plasterboard, just need the fire connected so I can get the chimney breast plastered and put in the Limestone firplace surround, I have already put in a new vent as advised by the fires paperwork.
I am going to get in a gas fitter to connect etc for this but want to try and keep the cost down.
The fire is to be fed by a 8mm?? pipe that has to go through the external wall and connect up to an existing 22mm pipe feeding my cooker hob and boiler from an exterior pipe. Total run would be about 3 metres.
Questions I have are:
1:
Will the gas fitter use 22mm pipe from the external feed already present and reduce down to 8mm in the fireplace, this would be best for me as I have a couple of 22mm pipes left over from when I had a new boiler fitted recently so if he used these hopefully he'd lower his price.

2:
What size hole will need to be drilled through the wall?
If he uses the 22mm pipe I guess it would need sleeving with 28mm pipe, am I correct in this? If so I will have the hole already drilled ready for him.

3:
I want all the gas feed to the fire to be out of view inside the false chimney breast, is this allowed if he put an isolator on the external feed to the fire which is in my back garden?

Your help greatly appreciated as always