Replacement gasket for flue through brickwork?

[sharpener]

I have a 20kW Vokera Mynute boiler with a horizontal 60/100 mm concentric flue going through an external brick wall.

The original flanged gasket which provides a neat finish to the hole and weather protection round the terminal has fallen apart comprehensively.

 

Vokera do not sell this separately, only as part of their complete terminal kits. Repair with flashing tape would be difficult as the job has to be done leaning over the edge of the bathroom roof.

 

Anyone suggest where I can get a suitable replacement?