YA sci fi novel with two warring alien races on Earth

Please help me to identify a book I read back in the 90s.

It's a young adult sci fi novel. I believe it's set in Britain. The MC is a teenage boy who suffers from painful, irregular growth spurts. He owns a dog named Winston. Early in the book, he learns that there are two alien races have made their home close to his town. They are in conflict with each other, each painting the other as the aggressor, and are locked into a stalemate due to numbers. Both races take a human appearance, but this is not their natural form.

One race is technologically gifted, fighting and hunting with their machines on land >!and in their natural shark-like form in the water!<. Jista, a girl with silvery hair who belongs to this faction, is the one who recognises and befriends the MC in the first few pages of the book and begins to reveal the secrets of this conflict to him. She takes him to her faction's fortress in the forest; here he meets her father, who appeals to him for help in the conflict, and her brother, to whom he takes an instant mutual dislike. Said brother keeps a pet great white shark in a massive aquarium. Jista's faction gifts the MC with a robot hummingbird to protect him if the other faction attacks him, >!though when he tests it, it explodes violently in a way that would have harmed many bystanders!<.

The second alien race possess a sylvan connection to nature and a kind of folk magic in addition to their true form. One of them, an abrasive girl with spiky red hair, becomes his guide to her faction. She has a furry cat-like alien pet which, in one scene, rapidly inflates to the size of a city bus when threatened. This faction's football-shaped homeworld, revealed to the MC by the redhead in a vision, drifts around the universe; currently it is moving away from Earth, and the sylvans are trying to contact their people there so they can be brought home. >!They fail and are stranded on Earth forever. It is revealed that the MC is only half-human, the son of the sylvan leader, Lord Darkwater(?), who had an affair with his mother; this mixed blood explains his anomalous growth spurts.!<

!In the end, having uncovered the rapacious nature of the technologists, the MC sides with the sylvans and is rescued from their fortress by the redhead. The MC discovers some of his hereditary magic during the battle, and he and the redhead are hunted through the forest as they escape back to her people. Lord Darkwater goes to the technologists to sue for peace(?) and is killed, as predicted. This breaks the stalemate and prompts the technologists to commit to a final attack. However, Darkwater's sacrifice was a ruse to prompt this reaction, as the MC takes his place in assisting in the sylvans' defense. Ultimately, the MC is able to help the sylvans overcome their enemies at a river barrier, where all the shark-like aliens have merged physically into a single being to attack from the water. The MC and the redhead come to terms with their differences (and similarities) and carry on as family and friends.!<

Some other specific details I recall:

• the MC flicking a maggot off Winston's nose after he sticks it in a fisherman's bait bucket
• Lord Darkwater entertaining human children by using his magic to conjure coloured mice (blue from the air, red from his own blood)
• the MC and the redhead switch pets for a while, each refusing to return to their owner
• the cover art was the MC and Jista in the foreground with the redhead a ways behind them, her cat-like pet draped around her neck and shoulders
• among other things, the redhead can run up trees, conjure berries, teleport (which she does right into the MCs room, seemingly just to annoy him), and is able to toss a technologist fighting machine around without touching it (telekinetically?)
• the MC recalls learning that his grandmother used to spray deodorant on her hair and insect spray under her arms (or something similar); >!this after learning that the technologists are unable to tolerate weakness, killing off their sick and elderly!<
• in one scene, the MC fills his parents' pillow cases with shaving cream for a laugh; his father, a school teacher with little sense of humour, complains that he won't be able to shave the next morning, and is told by his mother that he'll have 'designer stubble' and look like a rock star

I can't believe I can remember all this but not MC's name, the author, or the title of the book.