You might indeed want to try to find an agent in Madras. I'm about to book a flight for a wedding in Calcutta, and I'm being advised by both the bride and my travel agent to fly to Mumbai or Delhi (probably LH to Delhi as BD to Mumbai is way more expensive), and then book an internal flight to Calcutta through a travel agent in India, because buying a ticket in Britain will be far more expensive.

Related question: I can't find Mumbai on the drop-down menus for booking flights using my DC miles. Is it ever there?