Many probably know this already, but for those who don't, here goes.

I only discovered today that a 20mm conduit coupling fits perfectly into a 25mm round box. So should you be short of a 20mm through box on site which was the case with me today and you happen to have a 25mm one, just push in 2 of 20mm couplings and away you go.

It could save you a trip to the wholesaler.