Now that the longer worktop is nearly done i need to look at the sink area which is in a much worse condition, several areas are slightly rotten and has plenty of cracks.
I spent a fair bit of time flattening the top surface that had several humps, the underside wasn't too bad at all with the worse areas near the sink aperture. The smaller cracks got a little Osmo wood filler applied. Excellent stuff, drys quickly and sands easy.
For the section behind the tap i've decided just to flatten, fill and sand it. If i don't get satisfactory results with this method i may have to route out the tap section and scarf a piece in. Not difficult but time consuming and very messy.
After I flattened and sanded the top i put my attention in the bottom surface, i decided to seal the sink aperture and tap hole with yacht varnish and the underside with a 50/50 mix of varnish and oil. This will offer much better resistance to water ingress over just Osmo oil in an area that gets wet often. Next step will be to join the pieces together, smooth out the joint, sand everything and finish.