There is a screw on the backside of the flue (3 in all) that wont bite......this is flue to boiler screw??? does this mean anything to you?
Yes it means that you haven't fitted the flue correctly
I would not do such a thing, If i had found that the last awkward screw would not go in, I would strip the whole install apart just to find out why?
But not if it was someone elses install
.... I would say all is well and to manufactures spec..then pull out all my fancy tools and make I knew how to use them
Then quickly scoupe up my gear before 6pm and make a hasty exit..