Ever since it first went on sale 30 years ago, M&S’s Colin the caterpillar cake has become somewhat of a British institution. The chocolatey treat is a failsafe for all ages and occasions, from kids’ and adults’ birthday parties (David Beckham and Dame Judi Dench are big fans), to farewell work dos. In fact, it’s proved so popular that M&S has launched an entire range of spin-offs: Colin...