>I assume rendering is my only option - but some of the brick looks sealed and rather shiny, will render stick to this?
There is a render that will stick, it is called polymer render (around $55), it covers 8-10sqm. This is the only product we would recommend over a painted / shiny surface. Unfortunately, render is not really recommended on a fireplace, it could possibly crack due to the heating/cooling.
Maybe someone else here has a few ideas on what you could do.
Anyone had any success or failure stories with rendering a fireplace?