Time travel into the English past, a brother and sister who fight just like real kids do, an endearing bad-tempered magical creature- the Mouldiwarp- who speaks in a broad Sussex accent, a witch who is sorely misunderstood but beneficent, a narrative that jogs along from adventure to adventure in a manner that strongly suggests Nesbit is making it up as she goes along...
It has a sequel- Harding's Luck- and that's what I'm going to read next.