Sir Gadabout: Sir Gadabout and the Camelot Calamity by Martyn Beardsley
Morag and Demelza, Sir Gadabout's old adversaries, have taken over the pet shop in Camelot village. They have adopted elaborate disguises and are keeping the real owner under lock and key, intending to cause so much chaos at the Round Table that they can move in and install themselves as King and Queen, with their ridiculous claim to the throne backed up by doctored family trees. The knights at Camelot begin to realise that their pet food is having a strange effect, and that pets they have bought from the shop aren't all they expected; Sir Gadabout himself has a little lizard which grows rapidly into a crocodile and terrifies everyone. The cause of the problem is eventually traced back to the pet shop and our determined knight is sent to investigate, resulting in the usual mayhem.