I can't see anything obviously wrong with it, so I can't explain the weight difference. How does the thickness compare with a normal one? (Pictures side by side would be best.) Have you checked the calibration of the scales?
If all these check out, it is an anomaly. Whether it is worth more than usual is debatable - British collectors are not as much into "error" coins as the Americans, particularly when it is an error in the minting process rather than in the dies used. On top of that you have to persuade potential buyers that this is a significant error, which may be tricky when all you have to show is the weight. It may be the only way you can discover the value is to list it on Ebay at auction with all the information and pictures you can and see where it ends up. My guess is the best you may get is a few pounds.