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6 by 2 with 3 inch depth is a happy hour at most with a sledgehammer, thinking about your boss/in laws/accountant/Boris Johnson is helpful. Hit a corner about 6" in and work your way inwards.
If you are built to climb and not descend, a medium breaker of around 16kg will do the job efficiently in 15 minutes and cost about £20 to hire (use a chisel rather than a point).
Anyone looking to buy a breaker, I can recommend the Screwfix own brand one (Erbauer), I've had mine for 3 years of very frequent use and it won't die, currently £200.
Draper breaker is fairly cheap on Toolstation and looks ok from the reviews. Use it to death and return or sell on after?
Edit: probably worth hiring a full size one for a day actually