Guessing the old one under the casing sleeve is welded? How did the old flashing fail? This is how they are often done by plumbers https://steelandtube.co.nz/product/pur/publications/design/chimney-flashing-round-flue
Just looking at your photo, that doesn't look to be a masport flue. The cowl and angled part looks very different. Mine looks like the one in the main photo on this PDF https://www.masportheating.co.nz/LiteratureRetrieve.aspx?ID=139412 . Was there a reason why they did use the manufacturers one?