Advice please - depth of Gas Fire?

Saversue

I want to replace my old free-standing gas fire for a built-in one. I have a marble surround and hearth, but a horrible fire installed by the builders. Also have a pre-fabricated flu, but don't know what depth fire it will take. I think it needs a slimline, but they vary from the slimmest at 78mm to 130 mm. I want the deepest I can use as they look nicer.
Is there any way of finding out without paying someone to come out and remove the old fire and actually measure the gap?