I've read the following in other threads:

C. International to International transfer

After landing look for the board saying "International Transfers" (DO NOT go through immigration).

1. If you are on a single ticket and already have boarding pass in hand with "DT" (Direct Transfer) printed on it, walk past the transfer desks and go through transit security. Then you can proceed to the departure lounges and gates. Note: if your boarding pass does not have "DT" printed on it, you need to talk to the transfer desk, otherwise the I-I security checkpoint will likely not let you through and send you back to the transfer desk.

2. If you have separate tickets or don't have your onward boarding pass in hand, you have to go to the transfer desk where the airline staff will check documents and issue your onward boarding pass. You cannot check-in online or get an e-Boarding Passes for international flights out of India. Check-in generally opens 4 hours before and closes 60-75 minutes before departure. Then go through transit security as in point 1.

3. There are no priority security lanes in transit security so it is advisable to keep adequate time (>2 hrs) between connecting flights even if you are in F or J.

4. If you have separate tickets, in addition it is advisable to hold an Indian visa or e-visa as you may be denied boarding at origin otherwise.

5. A transit hotel for overnight stays (up to 24 hours) is available after transit security.

I'm travelling hand luggage only from ATH-IST-DEL in Oct with Turkish Airlines and then DEL-KAT on a separate ticket with Vistara. (3hr transfer time)
I'm returning KAT-DEL (5hr transfer time) and then DEL-CDG-NCL

I may not have my DEL-KAT boarding pass on leaving ATH

If I don't need pick up checked bags, can I just head to the transfer desk, get my boarding pass printed for Kathmandu and proceed from there.

I'm reluctant to pay for an Indian Visa if its not needed.

Any help, much appreciated!