I use a travel agent for these. Well worth the extra money. Expertflyer can be of some help finding space in the right booking classes, watch out for married segments (Two flights available separate but not together).
Also read the fare rules. If you run into married segements you can always add 'stopovers' and continue on the next flight out.

Did you look at RT tcikets to India which allow a stop in LHR on the way? Likely possible on airlines which have LHR as a hub.