I think I might be able to help there as my parents bought me this train pack and they got it brand new from a mail order catalogue (possibly the old Parks Catalogues I think it was?) and it might well have been a limited run exclusive, as I never saw it available anywhere else and it came with a certificate.
I think I got mine sometime around 2004, as Tangmere was hauling Excalibur Express charters after returning to steam from restoration in 2003. An Excalibur Express headboard (which I sadly misplaced) was also in the accessories bag along with the usual accessories and lamps and headcode discs).