Problem with balanced flue pipes?

Have just purchased a Kinder gas fire balanced flue model and the corgi engineer came to fit it this afternoon.He fitted the fire in the purpose built fireplace but after fitting the 90`degree elbow ontop of the combustion chamber he could not get the straight additional flue pipe , which goes through the cavity wall to the outside , to fit into the elbow,there appears to be no other way that it could fit into the elbow ,all other parts fit each other.we are both at our wits end ,any help PLEASE!!