Quick update:
Local authority pointed me to the GRO, GRO pointed me back to the local authority who then directed me to PAC-UK. For anyone else in a similar situation, here's their reply:
Thank you for your enquiry. We can help in this matter but do have to charge.
The law has only recently changed to allow you to ask for information from your father’s adoption file and it remains a relatively complicated business and might not result in you obtaining the information you are after.
This is particularly true for you as the adoption will have been among the first and the files may be difficult to find.
Initially, you would need to attend our Leeds or London offices to ascertain you are who you say you are and to understand the purpose of your search. We would then proceed to look for the records. If we found them we are required to look for birth relatives who then have the right to say you cannot have their identifying information. This means you could be told the story but not any names which is frustrating from your point of view.
Currently, our charge is £510 but it is being reviewed.
If you want more information or to make a referral please call our Advice Line on 020 7284 5879.