Renew Indian passport after very long time

Hi,

My father lives in the UK and has ILR. He has not applied for British citizenship and his Indian passport expired almost 35 years ago.

He never had a reason to leave the UK so didn't renew. Now, we are hoping to process a renewal.

I'm wondering if anyone here has ever been in a similar situation and could mention about their experience and offer any advice how best to go about it - i.e. what pitfalls to avoid and any tips would be greatly appreciated.

One example of an issue I've faced - on the online form it requests for a "File Id", which old passports did not have. How to overcome issues like this?

Kind regards