An Indian national will always need a visa to transit the UK unless they hold one of the exemption documents detailed above.
Do note though that Only a 'Common Format Residence Card' is accepted as an exemption document. Many residence cards issued in the EU are not 'Common Format'.
Also, only a Schengen Category D visa can be used as an exemption, most visitors are often only on a Category C visa.

As Often1 says the most widely accepted source is TIMATIC which is available to the public through the IATA travel Centre website.