That is NOT the access to the waste water tank, Tokkeh. That’s access to the blown air heater, the waste pipe from the basin and various wiring. (This is handy when doing any wiring round this area, as you can gain access to the area behind the tall cupboards, cooker and sink units)
To remove that panel is awkward, as it is VERY tight. The answer is to carefully insert a very thin pen knife blade (craft or Stanley knife blades are too thin and will most likely snap). Then gently wiggle the blade so that it pulls the panel gradually out. One of the rounded corners may give you better purchase on the edge of the wood. Pay attention that you’re not damaging the surrounding woodwork as you gradually lever it out. A little silicon grease or candle wax along these edges could make removal easier later.
Now, back to your problem with the waste tank… Access to this tank is via the bottom cupboard under the hob. Lift out the drawers by pulling right out, then while lifting, flip the little pointed pieces of plastic at the front of each runner – this will free the drawer from the runner and it can be removed. At the base, inside this cupboard, is a panel with a finget-pull hole in it. Removing this will give you access to the top of the tank. I know there is “wiring” there, but I’ve never investigated further. May be another member has further advice?
Good luck!
Barry (ex-Exsis!)