Reg. Search – Help!

I am currently in the process of compiling my “history of flight” in other words I am trying to make a list of all aircraft I have flown on.
I have been fairly successful, however I am stuck on one flight, which I must admit dates back quite some years!

Can anyone help me in tracing the reg. of the aircraft that operated the following flight:
20.12.1985 BA552 LGW-MLA

I don’t know if this helps, but it was a B737-200 and with British Airtours titles.

Thanks for any help!

Best regards