Whether £30K a year is enough to decorate a flat or not is not really the point, is it?
The point is that he took a bung to spend a lot more than that, didn't declare it, therefore owed the bunger a favour, it was leaked, he's trying to spin it that eight months later he just hasn't declared it yet, it should have been declared within six months MAX, and anyway he's paid the bung back (making it a "loan").
This wouldn't be much of a story if it wasn't yet another indicator of Boris's character in both his personal and professional life.