Just in case the OP, or anyone else is thinking of applying headcodes on model locos, just remember that BR ceased to display headcodes on trains around 10 years before Class 31 started to run regularly into Liverpool Lime Street. Their earliest appearances were on the CLC expresses to Manchester, Sheffield & beyond, and they finished on the Liverpool to Preston & beyond services, but they also appeared on Trans Pennine services, and less frequently on "Chat Moss" & "Crewe" stopping services. If you can access WTTs for the area from the mid 1980s to early 1995, that should identify some valid headcodes.