There are a several outfits in the UK that can recalbrate a speedo to fix this.
Be sure to ask, though, if they will also recalibrate the trip meter and odometer. Those are much harder to change, because they are gear-driven rather than just relying on the strength of a magnet. Some shops will agree to recalibrate the speedo and not change the other two.
A ratio adapter is probably easiest & cheapest, if your original speedo head is in good condition (and matches its original calibration) otherwise. It's been a few years ago, but a local speedo shop offered to build me a ratio adapter for $100 with any ratio I wanted. He also pointed out that later ratio changes would only be $40.
PS, if you really have a 4.1 speedo head, I might know someone that would like to buy it. They're apparently pretty rare, and I wouldn't sell him mine.