Jamesdixion 201
I have not been through Delhi transferring into International flight as have always been going to Delhi or internal Indian flight at worst (there Sept 2015). However as other,s have said you should be able to check through your bags from London to Colombo, and I would heartily endorse Paxboy comment re lounge access (do what ever you have to do to get it !), as you will find (depending on flight times ) IGI an "interesting" experience, and a glimpse of what "Incredible India" can offer ! Have to echo Andy S comments, why did you not stage through the Mid East ?,(my normal route) as there are quite attractive fairs ? Anyway enjoy the experience as the people are unfailingly courteous although organisation can sometimes be a little, shall we say, hidden.
Have a good trip but get lounge access if at all possible you have a long wait.
Regards
Mr Mac