"2 or 3 inches" oversized all round round is big enough to have weakened the wall and needs dealing with properly. (Did someone cut a ten-inch hole instead of a 10 cm one?) The correct remedy would depend on the construction of the wall. I certainly wouldn't let the original plumber bodge it up with sand and cement.
If it were my property, I'd start by seeing whether my home insurance could help or give advice. Next step would be a quote from a competent builder and plumber (whoever did that is not competent IMO) working together to remove the existing flu, make good the wall, probably by replacing a block and and few bricks and then install the flu properly.
Gas Safe may be able to help (they have a complaints scheme) but I'd still want any work done by the original installer to be supervised by someone appropriately qualified such as a building surveyor or structural engineer.