The new kitchen units are slightly lower than those that came out, the worktop slightly thinner and the sink is now an undermount, this has all led to the existing waste exit being relatively higher. The sink waste exits through a 40mm pipe that goes through the wall, but with the waste...
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Also check if the corners in your sink are square or rounded. I normally end up buying one slightly larger and open up the hole in the workbench to accommodate, assuming its made of timber and not granite etc etc. Although yours at 550mms will be limited in width given the width of ...