Chances are that it'll be a Stagecoach Cambus. If so then you'll need the registration number or the route number and time. Then write to:
Stagecoach in Cambridge
100 Cowley Road
You need to identify which council the taxi is registered with, Cambridge City or SCDC, and you will need with its registration number or plate number. Then for City Council registered taxis you need to write to:
Mr Jas Lally
Head of Environmental Services
4 Regent Street
And for SCDC you need to write to their taxi licensing office.