Coins of a slightly different style have been given to Tutbury on the basis of the mint reading. Essentially a crude bust of what is otherwise a normal Stephen penny with a voided cross and outward pointing lis in the angles. Your coin with the martlets in the angles was previously known only at Derby. There was a cut half of Tutbury on ebay about 2 years(?) ago. The reigns of Henry I, Stephen and the Anarchy are the least represented of all reigns in numismatic terms, with new mints, moneyers and even types recorded on a regular basis, an estimate that we know of only a third of the mint outputs in this period would not be too wide of the mark. e.g. Last week at the Timelines sale they had a Stephen penny from a previously unknown moneyer at a mint that wasn't known to be striking Stephen pennies (Wareham). Previously it was only known with coins of Matilda during this period. Two lots later was a coin attributed to Cirencester during the Anarchy. A generation ago, this mint wouldn't have even been suggested as a possible location. This period in history is quite fluid at the moment due to recent discoveries such as the Box hoard where previously unknown mints were attributed as a result of the contents. Your coin fills yet another hole in the jigsaw.