Hello, Geraint,
the buzzer is probably mounted on the fusebox/relay plate, behind the driver's side glovebox. I just looked and it's marked '36'. Mind you, the buzzer operates on the driver's door only, and through a different pair of contacts from the interior light, so the pillar switch is a possible suspect too. (It has 2 seperate 'poles' inside).
Yours, Davey.