What difference would there be in the rolling resistance between a tyre that is 28c and one that is 32c?

Idf inflated to about the same pressure, there is very little rolling resistance between a 28 and a 32 mm tire. In fact, on rougher surfaces, the wider tire will roll better, even at a slightly lower pressure. Of course a narrower tire is more aerodynamic, but if you aren't going much over 30 kph it won't matter. The only other area where a narrower tire has an advantage is weight. All this is uming that the 2 tires are of equal quality. A cheap, heavy 32 mm tire won't roll well, so if you decide to switch, make sure that they install top quality tires