Fitting new sink of slightly different size into existing worktop

You wouldn't be able to do this with filler either side.. the sink itself normally has a lip that sits on the top of the worktop to stop the sink falling through the opening .. You could try using and galvanized metal make up a cover if you like that could be put around the opening stuck to the worktop and over hangs the original opening by 20mm all around and that could work

I would never advise using filler as a means of making up a gap and I would prefer to see the sink to establish a fix. My first concern would be the measurements as the size given equates to a rectangle sink and not a circular inset. However, your dilemma would depend upon The type of worktop, and B how tightly the fitters cut around the existing sink as a 20mm lip is the usual cover for sink and you may be lucky and the 5mm would be covered. If the worktop is of a Corian type the aperture could be reconfigured. It is possible to have a thin stainless steel ring or frame constructed for the new sink to sit on if the above is not practical.

I would not recommend using a filler. However looking at the specs of the sink given, it looks like this is not a circular sink, but a rectangular one. I would probably use a stainless beading, if needed to fill in the gap, but it would all depend on the amount of gap that was left from the original cutout. Another thing that's omitted here, it the type of worktop, as this would make a big difference.

It's possible it will fit. 5mm difference in length is not significant & could be taken up by the normal flange around the sink. The depth would need to be widened to accommodate the bigger sink. You would need to check the radius of the curve. The best way to do this would be to purchase the sink & compare it against yours in situ. If you can measure the cut out in the worktop from underneath that may help too.