You could try the super glue (thin type, not the gel) and baking soda trick to fill it. Just fill the gap with the glue, you may have to do short sections so the glue doesn't cure around the edges. Sprinkle the area liberally with baking soda (bicarbonate of soda on your side of the pond). Don't be tempted to press it down with your finger, let the chemical reaction happen. Let it cure for 20-30 seconds, brush off the excess and there will be a rough surface that should be hard enough to begin sanding with your favourite files and grits of sandpapers and a final buff with a cotton cloth. If there are any gaps or depressions repeat the glue and baking soda steps as required.
Cheers,
RichB