TRANSIT WITHOUT VISA (TWOV):

A visa national who is going to the UK simply to travel on to another country may enter without a visa if these conditions are met:

• He arrives and departs by air
• His onward flight is confirmed and departs within 24 hours
• He is properly documented for his destination and has a visa if he needs one

TWOV is a concession which does not apply to nationals of the countries shown in red* who need visas even when in direct airside transit unless they are in possession of one of the following documents:

• A valid common format Category D visa for entry to an EEA state or Switzerland;

Germany is a part of EEA countries

*India is in the list of countries marked as red in the reference page.

Source :

http://www.ukba.homeoffice.gov.uk/sitecontent/documents/policyandlaw/carriers/ukvisarequirements.pdf

http://www.ukba.homeoffice.gov.uk/visas-immigration/transitthroughtheuk/visa-transit-airside/

This website is the official UK border agency website