[Arran]

I agree with Ron, I think that is exactly what happened, the original top came off, or suffered some other damage, so someone put a piece of thin plywood in it's place. The original may have been laminated too but thicker, and it had a tongue to fit into the groove on each side, the grooves that have since broken away. That cabinet needs a little more help then some wood filler and lacquer can offer on short notice. I would work on something else first that doesn't need so much reconstructive surgery.
Regards
Arran