Flying Goat, I went through this process last year. My original PPL was issued in 1969. I was informed by someone I considered to speak with authority that this would have made it a lifetime PPL, but, like you, I could not find the original to prove whether this was correct. However CAA insisted that my PPL, by its issue date, would have been a Board of Trade licence, and these were all of limited validity period. Maybe this is why I had not kept the original ?
For better or worse I opted for the new PPL and so far I have found it adequate for my needs. I agree that you can get differing opinions from within CAA and LASORS does not cover all scenarios. Let us know how you get on !