Hi, nice bike. If anything i'd swap that saddle for a Flight!
About Ribble. They exclusively built steel frames up until about 1995/6 before they went the mass-made taiwanese aluminium route. They ceased making steel models altogther in the early 2000's. The 531 ones were popular because they were half the price you'd pay for a 531 Dave Yates and such (the Ribble 531's were on sale for £200 according to my bike mag dated 1994). They built with 653 and 753 as well. They were generally well made with neat lug brazing.