I have got an e-ticket for the flight. It's Jet Airways, London to Colombo with a 7.5 hour connection in Mumbai. Same operator (Jet Airways). Do I need a transit visa? [EDIT: Is this the correct restatement of facts?]
Assuming the trip is in one booking, no, you do not need a transit visa.
Timatic, the database used by airlines, says:
TWOV (Transit Without Visa): Holders of confirmed onward tickets for a max. transit time of 24 hours
Not applicable to stateless persons and refugees.
So unless you're a refugee or stateless, you'll be fine.