I see this has been going on for some time. I will try to help you.
There is only one way to deal with this and that is by producing evidence of your date of birth.
The more evidence there is, the better.
You have already been told to provide a Statutory Declaration but I don’t know what was in the statutory declaration if indeed you did that. You might want to get it drafted by a solicitor therefore who would probably charge about £100. It would then need to be sworn in front of another solicitor who would charge a fixed fee of 5 pounds. It needs to explain that you were born in 1951 and that the time, in India, there were no birth certificates. You need to also explain that you have lost your Naturalisation certificate.
Before doing that, you might want to apply for a replacement certificate.
Here is the link
depending on when you were naturalised, depends on where you apply.
That then needs to be submitted to the Passport Office along with any other evidence. I suggest that you also get a letter from your GP who can confirm your date of birth.
If you submit that to the passport office and the passport office simply come back and say that it is not sufficient, then you make a complaint and as part of the complaint you ask them exactly what they require in order to correct (NB, correct, not “change”) your date of birth.
Here is a link to the passport complaint procedure
does that answer the question?
Can I answer any specific points arising from this?