Rats in the Cellar IV

This series has been about the many instances of the same boy, Oscar Wild Daugherty who got lost in the network of parallel worlds accessible via the two mysterious sets of stairs in their house. You thought that the series had come to a close. It has - almost.The time has come to shut down the parallel worlds portal in the big house on Oldhouse Road. Dr. James McSporran has arrived from the distant past to perform the task.
He asks Oscar Wild Daugherty, also known as Justin, to help him. But the task is far from straightforward. On shutting it down, they are flung into the bleak world that had resulted from the evil acts of McSporran's two unscrupulous pupils. There the house lies in ruins. Switching the portal back on only resets the device, which will now only work on the original world and any that would split off from that.
And split off, it does, resulting in two Oscars, and two Dr. McSporrans. At least Oscar will never be alone again, but how do they get home?The McSporrans find that two heads are better than one, but one of them must stay behind. The other takes the two boys. Their route takes them through Victorian England, where the unthinkable happens. The two Oscars are taken away to London into child labour, while McSporran is put in prison for trespassing in his own houseHow does one get back home when locked in a silk mill in Victorian London?