I was hoping Bob Jackson Cycles could help me to date the frame by referencing the serial number. I just read an email respons from BJC. For a mere twenty pounds sterling, they "can seek out the original build card and offer you a laser cut copy, this will give you all the geometry of the frame, the date it was manufactured the model name, a description of the original paint job and the name of the purchaser." £20 is a bit rich just to satisfy my curiosity, so I believe I shall instead happily live on in blissful ignorance as to the exact vintage! (For what it's worth, it appears to be a Grand Prix from the early 80's.)