UK Transit Visa

[hkg6755]

(I think I had earlier posted this under the US section)
Hi

I am planning to travel to India
and on return my flight is via London Heathrow(LHR).
It is JetAirways flight from Mumbai to London, then NorthWest flight from London to Detroit.

I have a valid USA H1 Visa till 2011.

I want to know if I require a UK transit visa just for
change of flight. The time between changing flights is about 3hrs

Your help is appreciated.

Thankyou