I don’t have any experience of Delhi Airport, but I’m 99% certain that, provided you have a through ticket (i.e. the Heathrow-Delhi and Delhi-Colombo sectors are on the same ticket), your bags will be checked through all the way to Colombo without you having to retrieve them.
I can’t think what possessed you to book a flight with a 13 hour layover though, let alone via Delhi. Personally I would have paid a little bit more for a civilised connection through one of the Middle Eastern hubs. I hope your tickets were a bargain for the journey you’re going to get!
As 22/04 notes, you WILL need a visa for Sri Lanka, but these are quick and easy to organise on-line before departure.