Oh, I forgot you might have wanted to actually know what had happened.
After encountering the vagabond magician and inviting him in the carriage, Tubbs talks with him about magic and about his suspicions that Starhouse was a fairie. The magician, sensing an easy mark, "enchants" an egg - one of those he had stolen from the villagers earlier - and sells it to Tubbs before departing. Tubbs is supposed to boil the egg and somehow trick Starhouse into eating it - which would have turned the coachman into a mouse. To return to human form, Starhouse would have had to use his own magical powers, thus proving that he is a fairie.
Starhouse knows nothing of this but he suspects his master and the magician are plotting something, and after seeing how smug the magician looks after they drop him off, Starhouse decides he's had enough. Late in the night he packs his stuff, swings by his friend the lock-keeper to say goodbye and disappears. The stuff Edward found in the loft - the boots, the coat and the other bits and bobs - are Tubbs property, used by the coachman but not actually belonging to him, so he's them left behind, not wanting to be accused of stealing.
Meanwhile, Tubbs has told his wife about his ingenious plan to force Starhouse to reveal his "powers". Mrs. Tubbs gets angry, leading to their evening row. In the morning Mrs. Tubbs dismisses the maid and makes the breakfast for her husband herself, meaning to feed him his own "enchanted" egg and prove to him that he's been swindled.
But! Dun dun dunnn! Turns out that the egg actually works! Mr. Tubbs turns into a mouse at the breakfast table. After a brief panicked scuffle Mr. Tubbs ends up being flung through the open window into the courtyard, being pursued by one of the farm cats until eventually escaping into the old well through one of the rotten boards. Mrs. Tubbs runs outside to save him, but not being able to find him falls ill with shock. She believes he's either died from the fall or that the cat has gotten him (or both); at the same time she hopes against hope that he's alive and hiding somewhere and that he'll come back when the spell eventually wears off. She blames herself for killing him and refuses to talk to anyone about it (as Max discovered).
Mr. Tubbs is actually alive somewhere in the well. Not being able to climb back up, he makes for the well in the back garden. The two are connected underground, but the one in the garden is even more dilapidated and Tubbs hopes the crumbled masonry would prove an easier climb. Unfortunately, turns out there are some creepy crawlies living in the cracks, blocking his way out, so he's been stuck there, surviving on fallen nuts and berries and digging himself even further in, trying to stay warm during the cold snap.