Now that I've repaired all the little bits that needed fixing and the top is once again flat, I got on with the sanding and finishing. First thing was to sand and seal the underside. Along with the other installation faults the worst was the underside not being sealed. Again to allow moisture to absorb and evaporate unevenly is to invite warping. Much better to get the underside done first so I'm less likely to damage the top surface by flipping it over after it's been sanded.
After the underside had dried I flipped it over for the last time. The front and end edges were given a small 45 chamfer and then sanded smooth to soften the profile slightly. The corner was rounded over and the end grain sanded very smooth.
A second and third coat will be applied once I sort out the other worktop and join it to this one. The sink worktop is in much worse condition.